Clean up command argument assembly

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.