Unnest rekey param parsing test and use ssh not sshd. ssh uses the same

Unnest rekey param parsing test and use ssh not sshd.  ssh uses the same
parsing code, now has "-G" to dump its config and is slightly faster
to start up.  This speeds up the test slightly (~5%) in the common case
but should help more during instrumented tests, eg under valgrind, where
startup costs are magnified.