First working support for arm (theoretically): changed some things:
SLACKVER and SLACKARCH can no longer be set by the user, instead,
SLACKVER will be constructed from SLACKMIRROR, as SLACKMIRROR now has
to point to the directory containing CHECKSUMS.md5, not the root directory
anymore.
Marked some options as x86-only: -v, -I and -n are more or less dependent on
x86/x86_64 Slackware mirrors. Instead of SLACKVER, SLACKMIRROR now gets checked
by chk_mir (was: chk_ver). Can't test arm functionality as I lack the hardware,
but should basically work.