A small pile of Windows compiler-warning fixes.

These include an unused variable left over from the command-line
refactoring; an explicit referencing of the module handle for
sspicli.dll which we really do deliberately load and then don't
(directly) use; a missing pointer-type cast in the Windows handle
socket code; and two 32/64 bit integer size mismatches in the types of
functions I was importing from system API DLLs.

The last of those are a bit worrying, and suggest to me that after
going to all that trouble to add type-checking of those runtime
imports in commit 49fb598b0, I might have only checked the resulting
compiler output in a 32-bit build and not a 64-bit one. Oops!

windows/winplink.c

@@ -297,7 +297,6 @@ int main(int argc, char **argv)
     int skcount, sksize;
     int exitcode;
     int errors;
-    int got_host = FALSE;
     int use_subsystem = 0;
     int just_test_share_exists = FALSE;
     unsigned long now, next, then;