Fix RETURN macros

Remove trailing semicolon

Fix RETURNNULL(X) -- was not executing X at all
Improve overlay buffer ob_xxx() primitives

All ob_append_xxx(b,...) functions return void

ob_makespace() returns 1 if successful, 0 if not

Add ob_overrun(b) predicate to check for overrun after any number of
appends
Add 'debug.overlaybuffer' logging

Instrument all ob_xxx() primitives with __whence
Improve MDP debug logging
Update 'dnaprotocol' debug flags