mount, umount: restore environ[] after suid drop

mount, umount: restore environ[] after suid drop

The commands mount and umount sanitize environment variables as it
works with suid permissions by default. Since v2.36 it's possible
that the commands drop the permissions and continue as regular user.
It seems we also need to restore the original environ to keep things
consistent for users (e.g. HOME=).

The implementation is pretty simple -- it keeps in memory removed
variables and use it after switch to non-suid mode.

Addresses: https://github.com/karelzak/util-linux/issues/880
Signed-off-by: Karel Zak <kzak@redhat.com>