regular.mk: re-added plymouth support to cinnamon

Thanks cow@ cinnamon is back in Sisyphus thus in regular builds;
the old gdm3/plymouth issue seems gone just as with gnome3 flavour.
base+regular: re-added mkfs.*

Whoops: XFS, JFS, NTFS, FAT support has been lost while dancing
with reusing rescue lists and back to being lean.

Thanks Vladimir Gusev for spotting [a part of] this.
regular.mk: care for pre-existing filesystems too

The installed livecd would lack fstab entries for the filesystems
other than those mounted explicitly during partition step; while
this might be considered either bug or feature, let's try that
and see.
live: extend user's group list with 'netadmin'

...so that they can run mtr, iftop and other goodies out-of-box;
thanks folks at forum.altlinux.org for a regular-icewm.iso
related feature request.
server.mk: minor refactoring

There was no real reason to use/cleanup/x11-alterator downstream
every time instead of promoting it upstream to the common point.
regular.mk: moved use/luks to .regular-base

Commit g2b975e1 got it wrong for server-regular as it turns out now,
at least until that image decides to support LUKS either.
install2: introduced +installer mixin

This one is a part of a larger rewrite to move away from
distro-centric build-up to configuration-centric one with
the particular packaging being of secondary importance
compared to actual functionality.
{base,desktop}+regular: move desktop bits from base

There were several packages which don't really belong to base
list but rather to the desktop one; given that both of these
are included in desktop images it's a no-op for them but the
server ones might be better off without graphics.
regular.mk: initial regular-server

A simple server image has been a pretty popular request
roughly always; thanks asy@ for specification draft.
install2: minor EFI related tweak

INSTALL2_PACKAGES turned out to be sensitive to the
feature addition order: if efi was added before install2
then the packages added by the former were overridden
by the latter.

This is also related to commit g7b76c73 as +installer
can be added pretty much anywhere, there's no warranty
that use/install2 appears early enough in configuration
build-up sequence.
0.9.15-alt1

- +installer
gear-store-tags