acct-*.eclass: Allow dynamic UID/GID assignment via -1

acct-*.eclass: Allow dynamic UID/GID assignment via -1

Allow a special value of '-1' to dynamically assign UID/GID for the user
or group.  This is intended to be used in overlays where proper
assignment does not take place but whose owners wish to switch to acct-*
packages.

While technically it is possible to choose a free UID/GID, it could be
taken afterwards by some Gentoo package and unnecessarily introduce
a conflict.  Using '999' was also suggested (as the first dynamic
UID/GID) but it would cause issues for people enabling
ACCT_*_ENFORCE_ID.  To avoid this, '-1' does not trigger collision
checks.

Signed-off-by: Michał Górny <mgorny@gentoo.org>