Sync backend/parser/scan.l with bin/psql/psqlscan.l.

Sync backend/parser/scan.l with bin/psql/psqlscan.l.

Make some minor formatting adjustments to make it easier to diff these
files and see that they indeed implement the same flex rules (at least
to the extent that we want them to be the same).

(Someday it'd be nice to make ecpg's pgc.l more easily diff'able too,
but today is not that day.)

Also run relevant parts of these files and psqlscanslash.l through
pgindent.

No actual behavioral changes here, just obsessive neatnik-ism.