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.

ssh2bpp.c

@@ -269,6 +269,7 @@ static void ssh2_bpp_enable_pending_compression(struct ssh2_bpp_state *s)
                           s->bpp.input_eof);                            \
         if (!success)                                                   \
             goto eof;                                                   \
+        ssh_check_frozen(s->bpp.ssh);                                   \
     } while (0)
 
 #define userauth_range(pkttype) ((unsigned)((pkttype) - 50) < 20)