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/fcntl/open.c

@@ -10,7 +10,12 @@ int open(const char *filename, int flags, ...)
 	va_start(ap, flags);
 	mode = va_arg(ap, mode_t);
 	va_end(ap);
-	return sys_open_cp(filename, flags, mode);
+
+	int fd = __sys_open_cp(filename, flags, mode);
+	if (fd>=0 && (flags & O_CLOEXEC))
+		__syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
+
+	return __syscall_ret(fd);
 }
 
 LFS64(open);