mount.cifs: treat uid=,gid=,cruid= options as name before assuming they're a number

mount.cifs: treat uid=,gid=,cruid= options as name before assuming they're a number

Sergio Conrad reported a problem trying to set up an autofs map to do
a krb5 mount. In his environment, many users have usernames that are
comprised entirely of numbers. While that's a bit odd, POSIX apparently
allows for it.

The current code assumes that when a numeric argument is passed to one
of the above options, that it's a uid or gid. Instead, try to treat the
argument as a user or group name first, and only try to treat it as a
number if that fails.

Signed-off-by: Jeff Layton <jlayton@samba.org>