Reflect the value of max_safe_fds in max_files_per_process

It is currently hard to figure out if max_safe_fds is significantly
lower than max_files_per_process. This starts reflecting the value of
max_safe_fds in max_files_per_process after our limit detection. We
still want to have two separate variables because for the bootstrap or
standalone-backend cases their values differ on purpose.

Author: JelteF

src/backend/storage/file/fd.c

@@ -1199,6 +1199,7 @@ set_max_safe_fds(void)
 {
 	int			usable_fds;
 	int			already_open;
+	char	   *max_safe_fds_string;
 
 	/*----------
 	 * We want to set max_safe_fds to
@@ -1214,6 +1215,16 @@ set_max_safe_fds(void)
 
 	max_safe_fds = Min(usable_fds, max_files_per_process);
 
+	/*
+	 * Update GUC variable to allow users to see if the result is different
+	 * than what the used value turns out to be different than what they had
+	 * configured.
+	 */
+	max_safe_fds_string = psprintf("%d", max_safe_fds);
+	SetConfigOption("max_files_per_process", max_safe_fds_string,
+					PGC_POSTMASTER, PGC_S_OVERRIDE);
+	pfree(max_safe_fds_string);
+
 	/*
 	 * Take off the FDs reserved for system() etc.
 	 */