Comment 29 for bug 941673

i'm going to push these patches to master. They're loosely based on ritz work in this bug, but they diverge in some significant ways:

1) i set a limit on the number of local users returned (ala comment 2)
2) i redid the wheel check to avoid needing an admin user cache
3) i cache shadow information a little differently and for a shorter period of time
4) i get "local user list" from /etc/shadow now instead of /etc/passwd (since /etc/passwd gets rsynced around)