Consistently use NULL for invalid GUC unit strings

Patch by Euler Taveira
docs: error for adding  _validated_ domains for existing uses

Report by David G Johnston
C comments: adjust execTuples.c for new structure

Report by Peter Geoghegan
doc:  mention more changes needed to use huge pages

Report by Laurence Parry
doc:  mention TM is ignored for to_date/to_timestamp()

Report by Goulven Guillard