This is a small code cleanup patch.
Several commands internally assemble command lines to call other
commands. This includes initdb, pg_dumpall, and pg_regress. (Also
pg_ctl, but that is different enough that I didn't consider it here.)
This has all evolved a bit organically, with fixed-size buffers, and
various optional command-line arguments being injected with
confusing-looking code, and the spacing between options handled in
inconsistent ways. This patch cleans all this up a bit to look clearer
and be more easily extensible with new arguments and options. We start
each command with printfPQExpBuffer(), and then append arguments as
necessary with appendPQExpBuffer(). Also standardize on using
initPQExpBuffer() over createPQExpBuffer() where possible. pg_regress
uses StringInfo instead of PQExpBuffer, but many of the same ideas apply.
