condition, man: Add support for ConditionSecurity=apparmor

Checking for the apparmor directory in securityfs means the apparmor module is
loaded and enabled, and hence should suffice as a test.

https://bugs.freedesktop.org/show_bug.cgi?id=63312

man/systemd.unit.xml

@@ -984,7 +984,8 @@
                                 may be used to check whether the given
                                 security module is enabled on the
                                 system.  Currently the only recognized
-                                value is <varname>selinux</varname>.
+                                values are <varname>selinux</varname>
+                                and <varname>apparmor</varname>.
                                 The test may be negated by prepending
                                 an exclamation
                                 mark.</para>

src/core/condition.c

@@ -162,6 +162,8 @@ static bool test_security(const char *parameter) {
         if (streq(parameter, "selinux"))
                 return is_selinux_enabled() > 0;
 #endif
+	if (streq(parameter, "apparmor"))
+		return access("/sys/kernel/security/apparmor/", F_OK) == 0;
         return false;
 }