autofs: best effort to maintain mounttab and mountdtab

When an automounted filesystem is successfully unmounted, call
rpc.umntall(8) with the -k flag.

rpc.umntall(8) is used to clean up /var/db/mounttab on the client and
/var/db/mountdtab on the server. This is only useful for NFSv3.

PR: 251906
Reviewed by: trasz
Differential Revision: https://reviews.freebsd.org/D27801