Rework the exit path of tls13_handshake_recv_action()

Rework the exit path of tls13_handshake_recv_action()

If an error occurs in action->recv() for a handshake that needs to
downgrade to legacy TLS, the artistic exit path led to hiding the
error under TLS13_IO_USE_LEGACY. Rework the exit path to be easier
to follow, preserving behavior except that the error can no longer
be masked.

Detailed analysis and initial diff by Masaru Masuda.
Fixes https://github.com/libressl/openbsd/issues/146

ok beck