support for php_serialize fixes #4153

Summary: This resolves #4153
Closes https://github.com/facebook/hhvm/pull/4158

Reviewed By: @sgolemon

Differential Revision: D1664260

Signature: t1:1664260:1415406608:4156e3a93123110a4a71c3fbe79eaa37c54d791c
Sendmail

Summary: Support ```sendmail_path``` in ```php.ini``` and use the standard sendmail path ```/usr/lib/sendmail``` rather than expecting it to be on the user's ```PATH``` which it often isn't.
Closes https://github.com/facebook/hhvm/pull/4138

Reviewed By: @sgolemon

Differential Revision: D1664106

Signature: t1:1664106:1415406718:669cbb275b7e2acdb2d5378f02a850b030cccf53