Preserve stdin/stdout when launching editor

Summary:
emacsclient requires that stdin be attached to a real pty device.
PHP's passthru() and system() functions connect the child processes'
stdin and stdout to pipes to capture the output, breaking emacsclient.

This change fixes the issue by using proc_open which allows allows for
explicit control of the childs fds.

Blame Rev:

Task ID: #

Reviewers: epriestley

Test Plan:
I used it to submit this diff.

DiffCamp Revision:

Revert Plan:

Database Impact:

Memcache Impact:

Other Notes:

EImportant:

• begin *PUBLIC* platform impact section -

Bugzilla: #

• end platform impact -

Differential Revision: 51