Silly API for do_pg_start_backup()

Why is it that do_pg_start_backup() expects its callers to pass in
an open DIR pointer for the pg_tblspc directory, rather than doing
the AllocateDir call for itself?  This complicates the callers, provides
no flexibility (since do_pg_start_backup() is hardwired to know that
pg_tblspc is what it was passed), and it doesn't seem to me to offer any
robustness either.  There are a couple of comments suggesting that
somebody thought that opening the directory early might be good, but
there's no defense of why, and I can't see that holding the directory open
across a checkpoint request is adding robustness; if anything more the
reverse, since it's a kernel resource we don't need during that phase.

Barring objection I'll remove the tblspcdir parameter and have
do_pg_start_backup() open and close the directory immediately
around the loop where it's read, like every other case in the
backend.

            regards, tom lane