man: rename systemd.conf to systemd-system.conf

Alias as systemd-user.conf is also provided. This should help
users running systemd in session mode.

https://bugzilla.redhat.com/show_bug.cgi?id=690868

src/core/system.conf

@@ -5,7 +5,7 @@
 #  the Free Software Foundation; either version 2.1 of the License, or
 #  (at your option) any later version.
 #
-# See systemd.conf(5) for details
+# See systemd-system.conf(5) for details
 
 [Manager]
 #LogLevel=info

src/core/user.conf

@@ -5,7 +5,7 @@
 #  the Free Software Foundation; either version 2.1 of the License, or
 #  (at your option) any later version.
 #
-# See systemd.conf(5) for details
+# See systemd-user.conf(5) for details
 
 [Manager]
 #LogLevel=info