upgrade-prep: don't remove boot entry if testing

It's pretty common for me to run the upgrade in test mode, see that
everything is working, and then run it for real. Removing the boot entry
makes that tougher than it needs to be, so leave it alone when we're
testing.
po: update POTFILES.in, add Makefile

I keep forgetting how to regenerate fedup.pot, so here we go.
sysprep: drop 'plymouth.splash=fedup'

As the comment says, we can drop this now that the plymouth patch is
upstream and built (see plymouth commit a9703fb).
fedup.sysprep: use upgrade-init when booting non-systemd

Add init_is_systemd(), and use that to check if we've got systemd or
SysV-style init.

If it's SysV, add 'init=/usr/libexec/upgrade-init' to the boot args to
start the upgrade on reboot.