util: add new safe_close_above_stdio() wrapper

At various places we only want to close fds if they are not
stdin/stdout/stderr, i.e. fds 0, 1, 2. Let's add a unified helper call
for that, and port everything over.

src/basic/fd-util.h

@@ -35,6 +35,13 @@ int close_nointr(int fd);
 int safe_close(int fd);
 void safe_close_pair(int p[]);
 
+static inline int safe_close_above_stdio(int fd) {
+        if (fd < 3) /* Don't close stdin/stdout/stderr, but still invalidate the fd by returning -1 */
+                return -1;
+
+        return safe_close(fd);
+}
+
 void close_many(const int fds[], unsigned n_fd);
 
 int fclose_nointr(FILE *f);

src/basic/log.c

@@ -94,14 +94,7 @@ static char *log_abort_msg = NULL;
         } while (false)
 
 static void log_close_console(void) {
-
-        if (console_fd < 0)
-                return;
-
-        if (console_fd >= 3)
-                safe_close(console_fd);
-
-        console_fd = -1;
+        console_fd = safe_close_above_stdio(console_fd);
 }
 
 static int log_open_console(void) {

src/basic/process-util.c

@@ -1428,8 +1428,7 @@ int fork_agent(const char *name, const int except[], unsigned n_except, pid_t *r
                         _exit(EXIT_FAILURE);
                 }
 
-                if (fd > STDERR_FILENO)
-                        close(fd);
+                safe_close_above_stdio(fd);
         }
 
         /* Count arguments */

src/basic/terminal-util.c

@@ -917,8 +917,7 @@ int make_stdio(int fd) {
         if (dup2(fd, STDERR_FILENO) < 0 && r >= 0)
                 r = -errno;
 
-        if (fd >= 3)
-                safe_close(fd);
+        safe_close_above_stdio(fd);
 
         /* Explicitly unset O_CLOEXEC, since if fd was < 3, then dup2() was a NOP and the bit hence possibly set. */
         stdio_unset_cloexec();