basic/fd-util: introduce stdio_unset_cloexec() function

There are some places in the systemd which are use the same pattern:

    fd_cloexec(STDIN_FILENO, false);
    fd_cloexec(STDOUT_FILENO, false);
    fd_cloexec(STDERR_FILENO, false);

to unset CLOEXEC for standard file descriptors. This patch introduces
the stdio_unset_cloexec() function to hide this and make code cleaner.

src/basic/fd-util.c

@@ -186,6 +186,12 @@ int fd_cloexec(int fd, bool cloexec) {
         return 0;
 }
 
+void stdio_unset_cloexec(void) {
+        fd_cloexec(STDIN_FILENO, false);
+        fd_cloexec(STDOUT_FILENO, false);
+        fd_cloexec(STDERR_FILENO, false);
+}
+
 _pure_ static bool fd_in_set(int fd, const int fdset[], unsigned n_fdset) {
         unsigned i;