Set max_safe_fds whenever we create shared memory and semaphores.

Formerly we skipped this in bootstrap/check mode and in single-user
mode.  That's bad in check mode because it may allow accepting a
value of max_connections that doesn't actually work: on platforms
where semaphores consume file descriptors, there may not be enough
free FDs left over to satisfy fd.c, causing postmaster start to
fail.  It's also not great in single-user mode, because fd.c will
operate with just the minimum allowable value of max_safe_fds,
resulting in excess file open/close overhead if anything moderately
complicated is done in single-user mode.  (There may be some penalty
for bootstrap mode too, though probably not much.)

Discussion: https://postgr.es/m/2081982.1734393311@sss.pgh.pa.us

src/backend/bootstrap/bootstrap.c

@@ -334,6 +334,12 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 
 	CreateSharedMemoryAndSemaphores();
 
+	/*
+	 * Estimate number of openable files.  This is essential too in --check
+	 * mode, because on some platforms semaphores count as open files.
+	 */
+	set_max_safe_fds();
+
 	/*
 	 * XXX: It might make sense to move this into its own function at some
 	 * point. Right now it seems like it'd cause more code duplication than

src/backend/tcop/postgres.c

@@ -4099,8 +4099,18 @@ PostgresSingleUserMain(int argc, char *argv[],
 	 */
 	InitializeWalConsistencyChecking();
 
+	/*
+	 * Create shared memory etc.  (Nothing's really "shared" in single-user
+	 * mode, but we must have these data structures anyway.)
+	 */
 	CreateSharedMemoryAndSemaphores();
 
+	/*
+	 * Estimate number of openable files.  This must happen after setting up
+	 * semaphores, because on some platforms semaphores count as open files.
+	 */
+	set_max_safe_fds();
+
 	/*
 	 * Remember stand-alone backend startup time,roughly at the same point
 	 * during startup that postmaster does so.