Add non-XS extensions to known_extensions.

Add non-XS extensions to known_extensions.

Previously "known_extensions" was misnamed, as it only contained known XS
extensions. grep.cpan.me suggests that there are only 10 mentions of it
outside the core, and none of them rely on this existing behaviour.

Update the descriptions of extensions, known_extensions and nonxs_ext in
Porting/Glossary.

These changes need replicating into configure.com.