Call ssh_check_frozen when BPP consumes input.

In commit 0f405ae8a, I arranged to stop reading from the SSH
connection if the in_raw bufchain got too big. But in at least some
tools (this bit me just now with PSCP), nothing actually calls
ssh_check_frozen again when the bufchain clears, so it stays frozen.

Now ssh_check_frozen is non-static, and all the BPP implementations
call it whenever they consume data from ssh->in_raw.

sshserver.c

@@ -76,6 +76,7 @@ void share_setup_x11_channel(ssh_sharing_connstate *cs, share_channel *chan,
 Channel *agentf_new(SshChannel *c) { return NULL; }
 bool agent_exists(void) { return false; }
 void ssh_got_exitcode(Ssh *ssh, int exitcode) {}
+void ssh_check_frozen(Ssh *ssh) {}
 
 mainchan *mainchan_new(
     PacketProtocolLayer *ppl, ConnectionLayer *cl, Conf *conf,