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.

sshcr.h

@@ -25,19 +25,19 @@
  *    Database for Edit and Continue'.
  */
 
-#define crBegin(v)      { int *crLine = &v; switch(v) { case 0:;
+#define crBegin(v)      do { int *crLine = &v; switch(v) { case 0:
 #define crBeginState    crBegin(s->crLine)
 #define crStateP(t, v)                          \
     struct t *s;                                \
     if (!(v)) { s = (v) = snew(struct t); s->crLine = 0; }      \
     s = (v);
 #define crState(t)      crStateP(t, ssh->t)
-#define crFinish(z)     } *crLine = 0; return (z); }
-#define crFinishV       } *crLine = 0; return; }
-#define crFinishFreed(z) } return (z); }
-#define crFinishFreedV   } return; }
-#define crFinishFree(z) } sfree(s); return (z); }
-#define crFinishFreeV   } sfree(s); return; }
+#define crFinish(z)     } *crLine = 0; return (z); } while (0)
+#define crFinishV       } *crLine = 0; return; } while (0)
+#define crFinishFreed(z) } return (z); } while (0)
+#define crFinishFreedV   } return; } while (0)
+#define crFinishFree(z) } sfree(s); return (z); } while (0)
+#define crFinishFreeV   } sfree(s); return; } while (0)
 #define crReturn(z)     \
         do {\
             *crLine =__LINE__; return (z); case __LINE__:;\