Assorted benign warning fixes.

These were just too footling for even me to bother splitting up into
multiple commits:

 - a couple of int -> size_t changes left out of the big-bang commit
   0cda34c6f

 - a few 'const' added to pointer-type casts that are only going to be
   read from (leaving out the const provokes a warning if the pointer
   was const _before_ the cast)

 - a couple of 'return' statements trying to pass the void return of
   one function through to another.

 - another missing (void) in a declaration in putty.h (but this one
   didn't cause any knock-on confusion).

 - a few tweaks to macros, to arrange that they eat a semicolon after
   the macro call (extra do ... while (0) wrappers, mostly, and one
   case where I had to do it another way because the macro included a
   variable declaration intended to remain in scope)

 - reworked key_type_to_str to stop putting an unreachable 'break'
   statement after every 'return'

 - removed yet another type-check of a function loaded from a Windows
   system DLL

 - and finally, a totally spurious semicolon right after an open brace
   in mainchan.c.

ssh.c

@@ -457,7 +457,8 @@ static void ssh_initiate_connection_close(Ssh *ssh)
     va_list ap;                                 \
     va_start(ap, fmt);                          \
     msg = dupvprintf(fmt, ap);                  \
-    va_end(ap);
+    va_end(ap);                                 \
+    ((void)0) /* eat trailing semicolon */
 
 void ssh_remote_error(Ssh *ssh, const char *fmt, ...)
 {