core: use fexecve() to spawn children

We base the smack/selinux setup on the executable. Let's open the file
once and use the same fd for that setup and the subsequent execve.

src/shared/exec-util.c

@@ -33,7 +33,6 @@
 assert_cc(EAGAIN == EWOULDBLOCK);
 
 static int do_spawn(const char *path, char *argv[], int stdout_fd, pid_t *pid) {
-
         pid_t _pid;
         int r;