newuidmap/newgidmap: added support for user matching by UID in /etc/sub[ug]id

Until now only exact username specification in /etc/sub[ug]id file allowed the
mapping. This prevented normal use for those users who use multiple usernames
with the same UID, as it rejected mapping even though it was allowed for
another username with the same UID.

This patch initially retains the old behaviour, for performance's sake. In the
first pass, new[ug]idmap only searches for exact username match.
If that yields no valid results, it continues into another loop, which does UID
resolution and comparison. If either definition (numeric UID mapping
specification or mapping specification for another username with the same UID as
current username) is found, it is used.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
sub[ug]id manpages: add note about performance when using login names versus UIDs

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
sub[ug]id: compare range before comparing username/UID, to avoid unnecessary syscalls

Change suggested by Nicolas François as performance optimization.
Performance penalty would be really noticeable when usernames are
stored in remote databases (ldap).

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>