simplify ctermid

opening /dev/tty then using ttyname_r on it does not produce a
canonical terminal name; it simply yields "/dev/tty".

it would be possible to make ctermid determine the actual controlling
terminal device via field 7 of /proc/self/stat, but doing so would
introduce a buffer overflow into applications built with L_ctermid==9,
which glibc defines, adversely affecting the quality of ABI compat.

src/unistd/ctermid.c

@@ -1,19 +1,7 @@
 #include <stdio.h>
-#include <fcntl.h>
-#include <unistd.h>
-#include <limits.h>
-#include "syscall.h"
+#include <string.h>
 
 char *ctermid(char *s)
 {
-	static char s2[L_ctermid];
-	int fd;
-	if (!s) s = s2;
-	*s = 0;
-	fd = open("/dev/tty", O_WRONLY | O_NOCTTY | O_CLOEXEC);
-	if (fd >= 0) {
-		ttyname_r(fd, s, L_ctermid);
-		__syscall(SYS_close, fd);
-	}
-	return s;
+	return s ? strcpy(s, "/dev/tty") : "/dev/tty";
 }