Currently only works for amd64 processors.

Wrap syscall.Wait4 and cleanup a few coordination issues.

There are still some issues here where background threads are left
sleeping. This could potentially cause weird issues. There are a few
more things I have planned to cleanup thread coordination issues.

Areas that need improving:

* Code cleanup
* Promote breakpoints back out of thread context
* Fix potential bug in "Next" implementation, when thread contexts
  switch

Fix bug involving detecting whether or not we have stepped into another
function when we plan on return from the function we are currently in.

Once the program detects that we have stepped into another function,
we simply calculate the return address and then set a breakpoint and
continue to that location, avoiding numerous syscalls.

Needs some refactoring and some optimization, but fixes several bugs.

Now that I'm using the step strategy, I put in an optimization where if
stepping into another function, simply find the return address, put a
breakpoint there, and then continue.

I decided to vendor all debug/dwarf and debug/elf files so that the
project can be go get-table. All changes that I am waiting to land in Go
1.4 are now captured in /vendor/debug/*.