Clean-up of all external hyperlinks.

* Link RFCs to the encrypted HTML version with extended functionality
compared to the TXTs.
* Link PHP Manual to the encrypted version which determines the user's
language and do not use the `www.` prefix since this leads to a
redirect.
* The PHPDoc link to version 1.4 does not work, use latest instead since
no other versions seem to work.
* Trailing slash for domain (index path)
* Link Wikipedia to the encrypted version.