app-shells/mksh: Fix the live ebuild

app-shells/mksh: Fix the live ebuild

 - mksh changed to tarballs so cpio and unpacker is now removed.
 - Update keywords from release ebuild
 - Rebuild.sh is useless for us
 - Bump to EAPI 7
 - Remove shell updater as mksh is in baselayout
 - Remove unstable -c lto
 - exeinto,doexe → into,dobin
 - Add test suite dependencies

 Closes: https://bugs.gentoo.org/show_bug.cgi?id=536992
 Closes: https://bugs.gentoo.org/show_bug.cgi?id=631948

Signed-off-by: Haelwenn (lanodan) Monnier <contact@hacktivis.me>
Signed-off-by: Michał Górny <mgorny@gentoo.org>