rpc.tlsclntd: Add an option to force use of TLS version 1.2

Commit 0b4f2ab0e913 fixes the krpc so that it can use TLS
version 1.3 for NFS-over-TLS, as required by
the draft (someday to be an RFC).
Since FreeBSD 13.0, 13.1 use TLS version 1.2 for
NFS-over-TLS mounts, this command line option
may be used so that NFS-over-TLS mounts to 13.0, 13.1
servers will still work.

Without the command line option, NFS-over-TLS mounts
will use TLS version 1.3.

The man page update will be a separate commit.

MFC after: 2 weeks