add support for "with"-subqueries into "join" & "sub-query" statements

Modifications to build HTML docs

Update from original

ALSO: fixed plan generation for "with"-subqueries

that I commented out in my last commit accidentally

fallout from the IF EXISTS changes

Add support for ALTER [ object ] IF EXISTS

Prevent NPE in IndexCondition.toString() that may arise from debugging

# Conflicts:
#	h2/src/main/org/h2/res/help.csv