os/os_defs.h cleanup: Remaining signal stuff

RETSIGTYPE, SIGRETURN, and SIGDEFARG are always defined the same, so
inline them for simplicity; SIGDUMMYARG, however, is unused, so remove
it.

Even if these things don't work on windows, the signal handler in
if_cscope.c is already wrapped with '#idef UNIX', as is its only call
site.

Reviewed-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/if_cscope.c

@@ -1829,10 +1829,9 @@ static int cs_read_prompt(int i)
 /*
  * Used to catch and ignore SIGALRM below.
  */
-static RETSIGTYPE
-sig_handler SIGDEFARG(sigarg) {
+static void sig_handler(int s) {
   /* do nothing */
-  SIGRETURN;
+  return;
 }
 
 #endif