rpc.tlsservd: Add an option to allow 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 mounts from 13.0, 13.1 will still work.

Without the command line option, only TLS version 1.3
mounts are permitted.

The man page update will be a separate commit.

MFC after: 2 weeks