The 'splash=0' parameter not only disables splash, but also switches
to tty5 on which plymouth is supposed to run. This looks like a bug
in propagator. 'nosplash' is also a general parameter for propagator
and make-initrd.

For install2, this is needed to work around error 99-cdrom.sh with method:cdrom.
The error shows up with make-initrd-bootchain, but not propagator, as lowmem is
enabled for propagator in this mode.
For rescue, the point is that the latter has become quite large, so by default
it would be better that way.

Turns out it's "splash=0" that does the job.
Suggested-by: Lenar Shakirov <snejok@altlinux.org>
See-also: https://bugzilla.altlinux.org/28625

This keyword was misused due to the lack of understanding
of its meaning; thanks zerg@ for the hint, let's hide the
kernel boot parameters not expected to be modified by user
under the hood (and vice versa in some cases).

Note that this is just a keyword shift in all cases
but mediacheck where "showopts" is now added.

NB: this keyword is only relevant for gfxboot.

"Forensic mode" submenu has fallen apart after the original commit
as the tricky logic in mkimage::tools/mki-copy-efiboot failed to
pick up the new variant; this should all be redone (solo@ has
started doing something but it needs a time-consuming review).

Fixes: 79d02088

This one is just like STAGE2_BOOTARGS but for rescue images
(which don't do that one for quite a few reasons).

This one provides cmdline arguments for startup-rescue >= 0.24
which would bring up networking and sshd in its turn thus allowing
remote access to the host booted in this mode.

The feature has been asked for by many people including mithraen@
and valintinr@ (and I'd make use of it another day too).

See the appropriate startup-rescue commit description for notes
on implementation; this default set of variable values should be
both useful and illustrative though.