add 'e' modifier (close-on-exec) to fopen and fdopen

this feature will be in the next version of POSIX, and can be used
internally immediately. there are many internal uses of fopen where
close-on-exec is needed to fix bugs.

src/stdio/__fdopen.c

@@ -4,7 +4,6 @@ FILE *__fdopen(int fd, const char *mode)
 {
 	FILE *f;
 	struct termios tio;
-	int plus = !!strchr(mode, '+');
 
 	/* Check for valid initial mode character */
 	if (!strchr("rwa", *mode)) {
@@ -19,7 +18,10 @@ FILE *__fdopen(int fd, const char *mode)
 	memset(f, 0, sizeof *f);
 
 	/* Impose mode restrictions */
-	if (!plus) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
+	if (!strchr(mode, '+')) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
+
+	/* Apply close-on-exec flag */
+	if (strchr(mode, 'e')) __syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
 
 	/* Set append mode on fd if opened for append */
 	if (*mode == 'a') {

src/stdio/fopen.c

@@ -17,6 +17,7 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
 	else if (*mode == 'r') flags = O_RDONLY;
 	else flags = O_WRONLY;
 	if (strchr(mode, 'x')) flags |= O_EXCL;
+	if (strchr(mode, 'e')) flags |= O_CLOEXEC;
 	if (*mode != 'r') flags |= O_CREAT;
 	if (*mode == 'w') flags |= O_TRUNC;
 	if (*mode == 'a') flags |= O_APPEND;