various: clean up isatty() handling

As per https://github.com/systemd/systemd/pull/30547#discussion_r1434371627

src/core/exec-invoke.c

@@ -670,12 +670,8 @@ static int chown_terminal(int fd, uid_t uid) {
         assert(fd >= 0);
 
         /* Before we chown/chmod the TTY, let's ensure this is actually a tty */
-        if (isatty(fd) < 1) {
-                if (IN_SET(errno, EINVAL, ENOTTY))
-                        return 0; /* not a tty */
-
-                return -errno;
-        }
+        if (!isatty_safe(fd))
+                return 0;
 
         /* This might fail. What matters are the results. */
         r = fchmod_and_chown(fd, TTY_MODE, uid, GID_INVALID);

src/core/execute.c

@@ -147,7 +147,7 @@ void exec_context_tty_reset(const ExecContext *context, const ExecParameters *p)
 
         const char *path = exec_context_tty_path(context);
 
-        if (p && p->stdin_fd >= 0 && isatty(p->stdin_fd))
+        if (p && p->stdin_fd >= 0 && isatty_safe(p->stdin_fd))
                 fd = p->stdin_fd;
         else if (path && (context->tty_path || is_terminal_input(context->std_input) ||
                         is_terminal_output(context->std_output) || is_terminal_output(context->std_error))) {