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
   0cda34c6

 - 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.