Improve pg_regress's error reporting for schedule-file problems.

Improve pg_regress's error reporting for schedule-file problems.

The previous coding here trashed the line buffer as it scanned it,
making it impossible to print the source line in subsequent error
messages.  With a few save/restore/strdup pushups we can improve
that situation.

In passing, move the free'ing of the various strings that are collected
while processing one set of tests down to the bottom of the loop.
That's simpler, less surprising, and should make valgrind less unhappy
about the strings that were previously leaked by the last iteration.