Minor fixups for psql's process_file() function.

- Avoid closing stdin, since we didn't open it.  Previously multiple
inclusions of stdin would be terminated with a single quit, now a separate
quit is needed for each invocation. Previous behavior also accessed stdin
after it was fclose()d, which is undefined behavior per ANSI C.

- Properly restore pset.inputfile, since the caller expects to be able
to free that memory.

Marti Raudsepp