Add REHASH SSLD (admins only) that starts new sslds and marks the
existing ones as inactive until all their clients disconnect.
Very useful whenever the SSL library has a vulnerability because
new connections can use a new version of the library without
disconnecting existing clients/servers.
Add STATS S (admins only) to list ssld processes, status, and client
count.
It's possible for which_ssld to fail and return NULL, handle this in
start_ssld_connect and start_ssld_accept by returning NULL. The NULL
return value is already handled in all calls to start_ssld_accept,
so handle this for start_ssld_connect by reporting an error connecting.
Handle it in start_zlib_session by exiting the client.
