Use negative_errno() to assert errno is positive after a few system calls

This is not particularly intrusive because it happens in simple
utility functions. It helps gcc understand that error codes
are negative.

This gets a rid of most of the remaining warnings.

src/basic/btrfs-util.c

@@ -2051,7 +2051,7 @@ int btrfs_subvol_get_parent(int fd, uint64_t subvol_id, uint64_t *ret) {
 
                 args.key.nr_items = 256;
                 if (ioctl(fd, BTRFS_IOC_TREE_SEARCH, &args) < 0)
-                        return -errno;
+                        return negative_errno();
 
                 if (args.key.nr_items <= 0)
                         break;

src/basic/clock-util.c

@@ -33,6 +33,7 @@
 #include "fd-util.h"
 #include "macro.h"
 #include "string-util.h"
+#include "util.h"
 
 int clock_get_hwclock(struct tm *tm) {
         _cleanup_close_ int fd = -1;
@@ -121,7 +122,8 @@ int clock_set_timezone(int *min) {
          * have read from the RTC.
          */
         if (settimeofday(tv_null, &tz) < 0)
-                return -errno;
+                return negative_errno();
+
         if (min)
                 *min = minutesdelta;
         return 0;

src/basic/path-util.c

@@ -102,7 +102,7 @@ int path_make_absolute_cwd(const char *p, char **ret) {
 
                 cwd = get_current_dir_name();
                 if (!cwd)
-                        return -errno;
+                        return negative_errno();
 
                 c = strjoin(cwd, "/", p, NULL);
         }

src/shared/ask-password-api.c

@@ -71,7 +71,7 @@ static int lookup_key(const char *keyname, key_serial_t *ret) {
 
         serial = request_key("user", keyname, NULL, 0);
         if (serial == -1)
-                return -errno;
+                return negative_errno();
 
         *ret = serial;
         return 0;