tty: write filename in a file, don't symlink to /dev

If the osquery tool[1] is running and scanning a directory containing
the `tty` symlink, it will follow the link to `/dev/ttys*` and start
some kind of infinite loop involving the debug console. This causes
hyperkit's CPU to spike and for the console itself to be spammed with
it's own output.

The easiest workaround is to not create the symlink, but write the name
of the tty in a file instead. Therefore for debugging we can still run

```
screen $(cat ~/Library/Containers/com.docker.docker/Data/vms/0/tty)
```
(or equivalent)

Reported originally around here[2]

[1] https://osquery.io/downloads/
[2] docker/for-mac#3499 (comment)

Signed-off-by: David Scott <dave@recoil.org>

Author: djs55

src/lib/uart_emul.c

@@ -758,14 +758,16 @@ uart_set_backend(struct uart_softc *sc, const char *backend, const char *devname
 
 			if (linkname) {
 				if ((unlink(linkname) == -1) && (errno != ENOENT)) {
-					perror("unlinking autopty symlink");
+					perror("unlinking autopty file");
 					goto err;
 				}
-				if (symlink(ptyname, linkname) == -1){
-					perror("creating autopty symlink");
+				int namefd = open(linkname, O_CREAT | O_WRONLY);
+				if (namefd == -1) {
+					perror("creating autopty file");
 					goto err;
 				}
-				fprintf(stdout, "%s linked to %s\n", devname, linkname);
+				(void) write(namefd, ptyname, strlen(ptyname));
+				(void) close(namefd);
 			}
 
 			sc->tty.fd = ptyfd;