spawn_sync: implement bindings
spawn_sync: closes pipes when killing inferior, cleanups

* If the child process is killed due to a buffer overflow or timeout,
  the stdio pipes are now closed immediately.

* Some logic was moved around and some methods/fields were renamed.
spawn_sync: report UV_ENOBUFS when output exceeds maxBuffer
spawn_sync: failure to start pipe i/o should be low-prio
spawn_sync: rename binding to just 'spawn'
child_process: initial stab at spawnSync binding
child_process: s/lookupSigno/lookupSignal/
process review comments
rebase on top of libuv api changes
rebase on top of multi-env
make multi-env actually work
handle null input better
add a couple tests
add some spawnSync docs
re-use same stdio validation logic
split spawn_sync declaration and definition
match builtin loader
lint child_process.js
share argument parsing between spawn()s
be more specific about allowed inputs