process: save original argv[0]

For historical and other reasons, node overwrites `argv[0]` on startup.
See
 - 2c6f79c0,
 - https://github.com/nodejs/node/issues/7434
 - https://github.com/nodejs/node/pull/7449
 - https://github.com/nodejs/node/pull/7696

For cases where it may be useful, save the original value of `argv[0]`
in `process.argv0`

PR-URL: https://github.com/nodejs/node/pull/7696


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>