selinux: split up mac_selinux_have() from mac_selinux_use()

Let's distuingish the cases where our code takes an active role in
selinux management, or just passively reports whatever selinux
properties are set.

mac_selinux_have() now checks whether selinux is around for the passive
stuff, and mac_selinux_use() for the active stuff. The latter checks the
former, plus also checks UID == 0, under the assumption that only when
we run priviliged selinux management really makes sense.

Fixes: #1941

src/shared/condition.c

@@ -231,7 +231,7 @@ static int condition_test_security(Condition *c) {
         assert(c->type == CONDITION_SECURITY);
 
         if (streq(c->parameter, "selinux"))
-                return mac_selinux_use();
+                return mac_selinux_have();
         if (streq(c->parameter, "smack"))
                 return mac_smack_use();
         if (streq(c->parameter, "apparmor"))