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