plugin_basename() fix

[Ozh]

Actual function plugin_basename() is broken and does not sanitize filenames on Win32 install. Therefore function register_(de)activation_hook() is also broken. Example : plugin_basename('c:\wordpress\wp-content\plugins\hello\hello.php') returns 'hello\hello.php'

Here is a fix for this function :

function plugin_basename($file) {
	$file = str_replace('\\','/',$file); // sanitize for Win32 installs
	$file = preg_replace('|/+|','/', $file); // remove any duplicate slash
	$file = preg_replace('|^.*/wp-content/plugins/|','',$file); // get relative path from plugins dir
	return $file;
}

Note that this will fix the problem of failed plugin activation hook, but will probably break some plugins relying on the broken function on win32 installs.

Dupe of #3002

Leaving open as has patch info.

Patch added to #3002 using this code.

Closing as dupe as the patch made it's way to #3002.