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: add execSync interface
test: add more exec and spawnSync coverage
doc: add spawnSync and execSync documentation