Don't limit subuid/subgid support to local users

Don't limit subuid/subgid support to local users

The current implementation of subuid/subgid support in usermod requires the
user to be a local user present in /etc/passwd.  There doesn't seem to be a
good reason for this; subuids should work equally well for users whose
records are in other NSS databases.

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1475749

Author: Steve Langasek <steve.langasek@ubuntu.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>