sysusers: look at login.defs when setting the default range to allocate users

sysusers: look at login.defs when setting the default range to allocate users

Also, even if login.defs are not present, don't start allocating at 1, but at
SYSTEM_UID_MIN.

Fixes #9769.

The test is adjusted. Actually, it was busted before, because sysusers would
never use SYSTEM_GID_MIN, so if SYSTEM_GID_MIN was different than
SYSTEM_UID_MIN, the tests would fail. On all "normal" systems the two are
equal, so we didn't notice. Since sysusers now always uses the minimum of the
two, we only need to substitute one value.