man: use external .c files for three examples

This way it's much easier to test that the code compiles without issues.
It's also easier to edit the code.

Indentation in one of the examples is reduced to two spaces. This is what we
use in man pages to make them fit on screen better.

man/journal-enumerate-fields.c

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: CC0-1.0 */
+
+#include <stdio.h>
+#include <string.h>
+#include <systemd/sd-journal.h>
+
+int main(int argc, char *argv[]) {
+  sd_journal *j;
+  const char *field;
+  int r;
+
+  r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
+  if (r < 0) {
+    fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
+    return 1;
+  }
+  SD_JOURNAL_FOREACH_FIELD(j, field)
+    printf("%s\n", field);
+  sd_journal_close(j);
+  return 0;
+}

man/journal-iterate-foreach.c

@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: CC0-1.0 */
+
+#include <stdio.h>
+#include <string.h>
+#include <systemd/sd-journal.h>
+
+int main(int argc, char *argv[]) {
+  int r;
+  sd_journal *j;
+  r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
+  if (r < 0) {
+    fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
+    return 1;
+  }
+  SD_JOURNAL_FOREACH(j) {
+    const char *d;
+    size_t l;
+
+    r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
+    if (r < 0) {
+      fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
+      continue;
+    }
+
+    printf("%.*s\n", (int) l, d);
+  }
+  sd_journal_close(j);
+  return 0;
+}

man/journal-stream-fd.c

@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: CC0-1.0 */
+
+#include <syslog.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <systemd/sd-journal.h>
+#include <systemd/sd-daemon.h>
+
+int main(int argc, char *argv[]) {
+  int fd;
+  FILE *log;
+  fd = sd_journal_stream_fd("test", LOG_INFO, 1);
+  if (fd < 0) {
+    fprintf(stderr, "Failed to create stream fd: %s\n", strerror(-fd));
+    return 1;
+  }
+  log = fdopen(fd, "w");
+  if (!log) {
+    fprintf(stderr, "Failed to create file object: %m\n");
+    close(fd);
+    return 1;
+  }
+  fprintf(log, "Hello World!\n");
+  fprintf(log, SD_WARNING "This is a warning!\n");
+  fclose(log);
+  return 0;
+}

man/sd_journal_enumerate_fields.xml

@@ -94,26 +94,7 @@
     <para>Use the <function>SD_JOURNAL_FOREACH_FIELD()</function> macro to iterate through all field names in use in the
     current journal.</para>
 
-    <programlisting>#include &lt;stdio.h&gt;
+    <programlisting><xi:include href="journal-enumerate-fields.c" parse="text" /></programlisting>
-#include &lt;string.h&gt;
-#include &lt;systemd/sd-journal.h&gt;
-
-int main(int argc, char *argv[]) {
-        sd_journal *j;
-        const char *field;
-        int r;
-
-        r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
-        if (r &lt; 0) {
-                fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
-                return 1;
-        }
-        SD_JOURNAL_FOREACH_FIELD(j, field)
-                printf("%s\n", field);
-        sd_journal_close(j);
-        return 0;
-}</programlisting>
-
   </refsect1>
 
   <refsect1>

man/sd_journal_next.xml

@@ -129,34 +129,7 @@
 
     <para>Iterating through the journal:</para>
 
-    <programlisting>#include &lt;stdio.h&gt;
+    <programlisting><xi:include href="journal-iterate-foreach.c" parse="text" /></programlisting>
-#include &lt;string.h&gt;
-#include &lt;systemd/sd-journal.h&gt;
-
-int main(int argc, char *argv[]) {
-  int r;
-  sd_journal *j;
-  r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
-  if (r &lt; 0) {
-    fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
-    return 1;
-  }
-  SD_JOURNAL_FOREACH(j) {
-    const char *d;
-    size_t l;
-
-    r = sd_journal_get_data(j, "MESSAGE", (const void **)&amp;d, &amp;l);
-    if (r &lt; 0) {
-      fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
-      continue;
-    }
-
-    printf("%.*s\n", (int) l, d);
-  }
-  sd_journal_close(j);
-  return 0;
-}</programlisting>
-
   </refsect1>
 
   <refsect1>

man/sd_journal_stream_fd.xml

@@ -100,33 +100,7 @@
     <para>Creating a log stream suitable for
     <citerefentry project='man-pages'><refentrytitle>fprintf</refentrytitle><manvolnum>3</manvolnum></citerefentry>:</para>
 
-    <programlisting>#include &lt;syslog.h&gt;
+    <programlisting><xi:include href="journal-stream-fd.c" parse="text" /></programlisting>
-#include &lt;stdio.h&gt;
-#include &lt;string.h&gt;
-#include &lt;unistd.h&gt;
-#include &lt;systemd/sd-journal.h&gt;
-#include &lt;systemd/sd-daemon.h&gt;
-
-int main(int argc, char *argv[]) {
-  int fd;
-  FILE *log;
-  fd = sd_journal_stream_fd("test", LOG_INFO, 1);
-  if (fd &lt; 0) {
-    fprintf(stderr, "Failed to create stream fd: %s\n", strerror(-fd));
-    return 1;
-  }
-  log = fdopen(fd, "w");
-  if (!log) {
-    fprintf(stderr, "Failed to create file object: %m\n");
-    close(fd);
-    return 1;
-  }
-  fprintf(log, "Hello World!\n");
-  fprintf(log, SD_WARNING "This is a warning!\n");
-  fclose(log);
-  return 0;
-}</programlisting>
-
   </refsect1>
 
   <refsect1>