Move MAC computations for Finished from ssl3_read_bytes into

Move MAC computations for Finished from ssl3_read_bytes into
ssl3_get_message, which is more logical (and avoids a bug,
in addition to the one that I introduced yesterday :-)
and makes Microsoft "fast SGC" less special.
MS SGC should still work now without an extra state of its own
(it goes directly to SSL3_ST_SR_CLNT_HELLO_C, which is the usual state
for reading the body of a Client Hello message), however this should
be tested to make sure, and I don't have a MS SGC client.