condition, man: Add support for ConditionSecurity=smack

According to Documentation/security/Smack.txt:
  In keeping with the intent of Smack, configuration data is minimal
  and not strictly required. The most important configuration step is
  mounting the smackfs pseudo filesystem.
This means that checking the mount point should be enough.

man/systemd.unit.xml

@@ -984,8 +984,9 @@
                                 may be used to check whether the given
                                 security module is enabled on the
                                 system.  Currently the only recognized
-                                values are <varname>selinux</varname>
-                                and <varname>apparmor</varname>.
+                                values are <varname>selinux</varname>,
+                                <varname>apparmor</varname>, and
+                                <varname>smack</varname>.
                                 The test may be negated by prepending
                                 an exclamation
                                 mark.</para>

src/core/condition.c

@@ -164,6 +164,8 @@ static bool test_security(const char *parameter) {
 #endif
 	if (streq(parameter, "apparmor"))
 		return access("/sys/kernel/security/apparmor/", F_OK) == 0;
+	if (streq(parameter, "smack"))
+		return access("/sys/fs/smackfs", F_OK) == 0;
         return false;
 }