tmpfiles: copy files to /etc only on boot

We'd copy /etc/nsswitch.conf, /etc/pam.d/, and /etc/issue (*) on every
tmpfiles --create run. I think we should only do this at boot, so if
people install systemd.rpm in a larger transaction and want to create those
files at a later step, we don't interfere with that.

(Stuff like /etc/os-release and /etc/mtab is not really configurable,
we might as was create it uncondtionally.)

(Seemingly, the alternative approach might be to not call
systemd-tmpfiles --create in systemd.rpm %post. But this wouldn't have much
effect, because various packages call it anyway, and our
%tmpfiles_create_package macro does too.  So we need to change the
configuration instead.)

(*) We don't provide /usr/share/factory/issue, so normally this fails, but
somebody else might provide that file, so it seems useful to keep the
C line.

tmpfiles.d/etc.conf.m4

@@ -15,8 +15,8 @@ t /etc/mtab - - - - security.SMACK64=_
 m4_ifdef(`ENABLE_RESOLVE',
 L! /etc/resolv.conf - - - - ../run/systemd/resolve/stub-resolv.conf
 )m4_dnl
-C /etc/nsswitch.conf - - - -
+C! /etc/nsswitch.conf - - - -
 m4_ifdef(`HAVE_PAM',
-C /etc/pam.d - - - -
+C! /etc/pam.d - - - -
 )m4_dnl
-C /etc/issue - - - -
+C! /etc/issue - - - -