upstream: more strictly enforce KEX state-machine by banning packet

types once they are received. Fixes memleak caused by duplicate
SSH2_MSG_KEX_DH_GEX_REQUEST (spotted by portable OpenSSH kex_fuzz via
oss-fuzz #30078).

ok markus@

OpenBSD-Commit-ID: 87331c715c095b587d5c88724694cdeb701c9def
