Add support to libtls for client-side TLS session resumption.

Add support to libtls for client-side TLS session resumption.

A libtls client can specify a session file descriptor (a regular file
with appropriate ownership and permissions) and libtls will manage reading
and writing of session data across TLS handshakes.

Discussed at length with deraadt@ and tedu@.

Rides previous minor bump.

ok beck@