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
handle null input better
spawn_sync: split declaration and definition
child_process: re-use stdio validation logic
child_process: spawn and spawnSync share parsing

argument and option parsing should be shared among spawn
implementations
child_process: add spawnsync test cases
spawn_sync: add pid to result object
child_process: spawnSync should not throw
spawn_sync: fix inherit stdio
child_process: execFileSync execSync interface
test: add more exec and spawnSync coverage
doc: add spawnSync and execSync documentation