tmpfiles: copy files to /etc only on boot

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.