Manual: Updated the compatibility chapter.
Manual: Updated compatibility chapter for Pike 8.1 and later.

Removes the section about Pike 7.6.

Adds a place-holder section about Pike 8.0.
Build: Fixed warnings about unused arguments in threads.c.
Build: Fixed some warnings in CritBit.
Build: Added PIKE_UNUSED_ATTRIBUTE.
precompile: Reduce warnings in the generated code.
Reinstate legacy XML parsing modes until further notice.
Reinstate legacy XML parsing modes until further notice.