Set UID_MIN to 1001 to make sure that useradd does not automatically add users with UID 1000.

This guarantees that UID 1000 is available when post-common adds the main user at the end of the first run of crouton.

Protect GID 1000 in the same way.

Fixes a bug when xbmc is selected during the Arch install (the xmbc user created by the package steals UID 1000).