Fix various TempFile issues

Summary:
This class is a mess, I think it hasn't been touched since it came out of FB.

• $dir parameter to constructor does nothing.
• preserve() does nothing.
• Fix documentation.
• Use more Filesystem.

Test Plan: Used setPreserveFile() in a future diff.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2996