Do not use realpath() on includes.

If the file does not exist, realpath() returns false and "include false;"
produces "Failed opening '' for inclusion" which is a useless error message.

'include' works just fine with symlinks, "./" and "../".
Also remove other unnecessary realpath() calls.
Use __DIR__ instead of dirname(__FILE__).

This is possible because we require PHP 5.3 or higher.
Merge pull request #4537 from owncloud/no-realpath

Do not use realpath() on includes.