The filename found in the URL will be used for old plugins missing a
base entry in their plugin.info.txt and lacking a subdirectory inside
the archive as well. This patch makes sure possible query strings aren't
included in the filename.

Note: io_download() will also try to get a filename from any
content-disposition header.

If no filename can be found we simply use an md5 sum of the URL and hope
the plugin will contain it's own hint for naming.

PHP_EOL is platform dependent, so you get in trouble while migrating
between platforms.

they were just catching and rethrowing

This reads some duplication in the from of haveing a Doku_Parser_Mode
and Doku_Parser_Mode_Plugin class which are basically the same but only
the latter extends DokuWiki_Plugin. This avoids the performance/RAM
problems mentioned in my previous commit. An interface keeps both
logically together.

With PHP 5.4 further deduplication could be done via Traits.

This makes Doku_Parser_Mode inherit from DokuWiki_Plugin which allows
for the removal of a bunch of duplicate code form
DokuWiki_Syntax_Plugin. This makes the code easier to maintain and makes
sure all DokuWiki plugins are actual instances of DokuWiki_Plugin.

However this adds a bunch of functions to the "normal" parser modes that
don't need them which could have performance/RAM implications.

this is not a extension specific cache but a global one. no need to
purge for each installed extension

The plugin manager echos raw URLs in error messages, this could allow to
construct an XSS attack. However the affected form is CSRF protected,
so an attacker would require another XSS vulnerability to get the needed token,
rendering this attack unneeded. So this should not be exploitable.