su: clean up groups initialization

su: clean up groups initialization

This patch does not change any su/runuser behaviour, code changes:

* don't use huge groups[NGROUPS_MAX]; the array has 256k, but we need
  it only occasionally when -G/-g specified.

* the current code uses groups[0] for -g and the rest for -G, this patch adds
  'gid' to remember -g argument to avoid memmove()

* add function add_supp_group() to simplify su_main()

* add note about -G and -g relation to the man pages (undocumented now)

Signed-off-by: Karel Zak <kzak@redhat.com>