Userspace fexec

Implements fork and execve here instead of in the kernel.

src/platform/redox/clone.rs

@@ -228,6 +228,12 @@ fn fork_inner(initial_rsp: *mut usize) -> Result<usize> {
     // Unblock context.
     syscall::kill(new_pid, SIGCONT)?;
 
+    // XXX: Killing with SIGCONT will put (pid, 65536) at key (pid, pgid) into the waitpid of this
+    // context. This means that if pgid is changed (as it is in ion for example), the pgid message
+    // in syscall::exit() will not be inserted as the key comparator thinks they're equal as their
+    // PIDs are. So, we have to call this to clear the waitpid queue to prevent deadlocks.
+    let _ = syscall::waitpid(new_pid, &mut 0, syscall::WUNTRACED | syscall::WCONTINUED);
+
     Ok(new_pid)
 }
 #[no_mangle]