pgsql: Make SET synchronous to avoid surprises with subsequent queries.
pgsql: Simplify regexps, add docs.
pgsql: Case insensitive regexps, but more readable.
pgsql: Make better use of the Stdio.Buffer add/add_ints API.
pgsql: FETCH is eligible for parallelisation too.
Stdio.Buffer: LFUN::_sprintf() must not have side effects!

sprintf("%s", buf) and sprintf("%q", buf) had the side effect
of emptying the buffer (which is not what you expect when you
use them for debug output...).