Handle group-stop in Parrot.

This branch moves to using PTRACE_SEIZE (for Linux 3.4.0+) to enable handling group-stop.

Interesting tidbits:

* execve no longer has the extra SIGTRAP when using PTRACE_SEIZE. Right now ignoring it looks a little awkward code-wise. It could maybe be cleaned up.
* There is a new PFS_PROCESS_STOP_STATE.
* We now correctly wake parents waiting for continued children.

This may also fix the lost thread issue.

This branch needs some testing on older kernels before merging. A test case would be nice too.