improve error handling of ttyname_r and isatty

POSIX allows ttyname(_r) and isatty to return EBADF if passed file
descriptor is invalid.

maintainer's note: these are optional ("may fail") errors, but it's
non-conforming for ttyname_r to return ENOTTY when it failed for a
different reason.

src/unistd/isatty.c

@@ -1,9 +1,13 @@
 #include <unistd.h>
+#include <errno.h>
 #include <sys/ioctl.h>
 #include "syscall.h"
 
 int isatty(int fd)
 {
 	struct winsize wsz;
-	return !__syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz);
+	unsigned long r = syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz);
+	if (r == 0) return 1;
+	if (errno != EBADF) errno = ENOTTY;
+	return 0;
 }

src/unistd/ttyname_r.c

@@ -9,7 +9,7 @@ int ttyname_r(int fd, char *name, size_t size)
 	char procname[sizeof "/proc/self/fd/" + 3*sizeof(int) + 2];
 	ssize_t l;
 
-	if (!isatty(fd)) return ENOTTY;
+	if (!isatty(fd)) return errno;
 
 	__procfdname(procname, fd);
 	l = readlink(procname, name, size);