accesslib: build_context_path() - better UPDATE SQL

accesslib: build_context_path() - better UPDATE SQL

A bit of cleanup on the SQL we use for updates. Unfortunately, MySQL
uses non-SQL-standard syntax for updates reading or updating multiple
tables. After much testing across Pg and MySQL, I boiled it down to
the minimal differences -- but we still have different SQL.

The timings for this on a large reference install, calling
build_context_path(true):

 - MySQL incompatible 1s
 - Petr's fixes 60s -- DB complains of too many writes
 - This commit 24s  -- DB complains of too many writes

Also - fixed a remaining problem with $emptyclause

Affects MDL-11347