Move all extern declarations into header files.

This is another cleanup I felt a need for while I was doing
boolification. If you define a function or variable in one .c file and
declare it extern in another, then nothing will check you haven't got
the types of the two declarations mismatched - so when you're
_changing_ the type, it's a pain to make sure you've caught all the
copies of it.

It's better to put all those extern declarations in header files, so
that the declaration in the header is also in scope for the
definition. Then the compiler will complain if they don't match, which
is what I want.

ssh2connection.c

@@ -325,13 +325,6 @@ static bool ssh2_connection_filter_queue(struct ssh2_connection_state *s)
     ChanopenResult chanopen_result;
     PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
 
-    /* Cross-reference to ssh2transport.c to handle the common packets
-     * between login and connection: DISCONNECT, DEBUG and IGNORE. If
-     * we have an instance of ssh2transport below us, then those
-     * messages won't come here anyway, but they could if we're
-     * running in bare ssh2-connection mode. */
-    extern bool ssh2_common_filter_queue(PacketProtocolLayer *ppl);
-
     while (1) {
         if (ssh2_common_filter_queue(&s->ppl))
             return true;