mount.cifs: make return codes match the return codes for /bin/mount (try #3)

mount.cifs: make return codes match the return codes for /bin/mount (try #3)

The manpage for /bin/mount specifies that the return code should be a
positive integer (actually, it's a bitfield). Clean up the return
codes from mount.cifs to make them match the expected return values
from /bin/mount. This necessary for proper integration with autofs.

This is the third attempt at this patch. The changes here are minor,
just changing some return's from main() into exit() calls for
consistency's sake.

Signed-off-by: Jeff Layton <jlayton@redhat.com>