Remove wfDl() and cleanup everything it touched

Remove wfDl() and cleanup everything it touched

wfDl() is a wrapper around dl(), which is an evil function and
basically only works from the command line of Zend. Luckily
no extension has ever used this thing, so let's just remove it
outright.

For comparison, here's a list of places it does not work:
- hhvm
- php as apache module
- php compiled with zts support
- safe_mode
- Basically any shared host that cares about security

Most callers are using it to check for extension support and are
actually failing gracefully when wfDl() returns false. In these
places we're just going to use extension_loaded().

While we're at it, clean up some of the test skip logic in the
media tests so we can bail as early as possible if we know we
can't complete the test.

This also immediately removes $wgLoadFileinfoExtension. It's been
enabled by default since 5.3 and falls back gracefully when the
support isn't available.

Change-Id: Ieb430dfc74483731dde51d6e20fa700d641ba1f4