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);