The beginnings of HipHop compiled mode support. It works now for parser cache hits.

The beginnings of HipHop compiled mode support. It works now for parser cache hits.

* Work around HipHop issue 314 (volatile broken) and issue 308 (no compilation detection) by adding some large and ugly compilation detection code to WebStart.php and doMaintenance.php.
* Provide an MW_COMPILED constant which can be used to detect compiled mode throughout the codebase.
* Introduced wfIsHipHop(), which detects either compiled or interpreted mode. Used this to work around unusual eval() return value in eval.php.
* Work around lack of ini_get() in Maintenance.php, by duplicating wfIsHipHop().
* In Maintenance::shouldExecute(), accept "include" as an inclusion function name, since all kinds of inclusion give this string in HipHop.
* Introduced new class MWInit, which provides some static functions in the pre-autoloader environment.
* Introduced MWInit::compiledPath(), which provides a relative path for invoking a compiled file, and MWInit::interpretedPath(), which provides an absolute path for interpreting a PHP file. Used these new functions in the appropriate places.
* When we are running compiled code, don't include files which would generate duplicate class, function or constant definitions. Documented the new requirements on the contents of Defines.php and UtfNormalDefines.php.
* In HipHop compiled mode, it's not possible to have executable code in the same file as a class definition.
  ** Moved MimeMagic initialisation to the constructor.
  ** Moved Namespace.php global variable initialisation to Setup.php.
  ** Moved MemcachedSessions.php initialisation to the caller in GlobalFunctions.php.
  ** Moved Sanitizer.php constants and global variables to static class members. Introduced an accessor function for the attribs regex, as a new place to put code formerly at file level.
  ** Moved Language.php initialisation of $wgLanguageNames to Language::getLanguageNames(). Removed the global variable, marked "private" since forever.

* In two places: don't use error_log() with type=3 to append to a file, HipHop doesn't support it. Use file_put_contents() with FILE_APPEND instead.
* Work around the terrible breakage of class_exists() by using MWInit::classExists() instead in various places. In WebInstaller::getPageByName(), the class_exists() was marked with a fixme comment already, so I replaced it with an autoloader solution.