This implements the user-facing APIs that lets one run a child process
and block until it exits.
Logic shared with the async counterpart of each function was refactored
to enable code reuse.
Docs and tests are included.
With -e or --eval, require() can load module using relative path.
node -e 'require("./foo")'
But it can't load module from node_modules directory.
node -e 'require("foo")'
Fixes#1196.