userns: Add negative depends on entries to avoid building code that is userns unsafe

Add a new internal Kconfig option UIDGID_CONVERTED that is true when the selected
Kconfig options have been converted to be user namespace safe, and guard
USER_NS and guard the UIDGID_STRICT_TYPE_CHECK options with it.

This keeps innocent kernel users from having the choice to enable
the user namespace in the cases where it is known not to work.

Most of the rest of the conversions are simple and straight forward but
their sheer number means it is good not to count on having them all done
and reviwed before thinking of merging this code.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>