Avoid defining SIGTTIN/SIGTTOU on Windows.

Setting them to SIG_IGN seems unlikely to have any beneficial effect
on that platform, and given the signal numbering collision with SIGABRT,
it could easily have bad effects.

Given the lack of field complaints that can be traced to this, I don't
presently feel a need to back-patch.

Discussion: https://postgr.es/m/5627.1542477392@sss.pgh.pa.us

src/backend/postmaster/postmaster.c

@@ -654,8 +654,12 @@ PostmasterMain(int argc, char *argv[])
 	 * a standalone backend, their default handling is reasonable.  Hence, all
 	 * child processes should just allow the inherited settings to stand.
 	 */
+#ifdef SIGTTIN
 	pqsignal(SIGTTIN, SIG_IGN); /* ignored */
+#endif
+#ifdef SIGTTOU
 	pqsignal(SIGTTOU, SIG_IGN); /* ignored */
+#endif
 
 	/* ignore SIGXFSZ, so that ulimit violations work like disk full */
 #ifdef SIGXFSZ

src/include/port/win32_port.h

@@ -171,8 +171,6 @@
 #define SIGTSTP				18
 #define SIGCONT				19
 #define SIGCHLD				20
-#define SIGTTIN				21
-#define SIGTTOU				22	/* Same as SIGABRT -- no problem, I hope */
 #define SIGWINCH			28
 #define SIGUSR1				30
 #define SIGUSR2				31