Add a setup warning about missing 'fileinfo'

Summary:
See <https://github.com/facebook/phabricator/issues/320>. We have a soft dependency on 'fileinfo', which we try to recover from (with `file`) but won't be able to on Windows and apparently FreeBSD systems. Since users can ignore setup checks anyway now, just raise a warning during install.

I believe almost all installs should have this extension, it has been part of the core for a long time.

Test Plan: Faked setup failure, looked at warning. "Solved" setup failure, saw it go away.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5952