Fix a double free when calling proxy_disconnect() inside phb->func()
Fixes trac ticket #1248
proxy_connected() calls phb->func(), then tries to do phb_free() directly
afterwards, but that might have been freed by a proxy_disconnect() call
during the execution of that callback.
This one happened to several different people because some AIM server
broke recently.
This commit fixes it by implementing a phb_connected() function that
removes the PHB from the hash table before calling phb->func(), which
ensures that any proxy_disconnect() calls just close the fd and nothing
else.
