add O_CLOEXEC fallback for open and related functions

since there is no easy way to detect whether open honored or ignored
the O_CLOEXEC flag, the optimal solution to providing a fallback is
simply to make the fcntl syscall to set the close-on-exec flag
immediately after open returns.

src/stdio/__fopen_rb_ca.c

@@ -8,6 +8,7 @@ FILE *__fopen_rb_ca(const char *filename, FILE *f, unsigned char *buf, size_t le
 
 	f->fd = sys_open(filename, O_RDONLY|O_CLOEXEC);
 	if (f->fd < 0) return 0;
+	__syscall(SYS_fcntl, f->fd, F_SETFD, FD_CLOEXEC);
 
 	f->flags = F_NOWR | F_PERM;
 	f->buf = buf + UNGET;

src/stdio/fopen.c

@@ -20,6 +20,8 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
 
 	fd = sys_open_cp(filename, flags, 0666);
 	if (fd < 0) return 0;
+	if (flags & O_CLOEXEC)
+		__syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
 
 	f = __fdopen(fd, mode);
 	if (f) return f;