Run check_keywords.pl anytime gram.c is rebuilt.

This script is a bit slow, but still it only takes a fraction of the time
the bison run does, so the overhead doesn't seem intolerable.  And we
definitely need some mechanical aid here, because people keep missing
the need to add new keywords to the appropriate keyword-list production.

While at it, I moved check_keywords.pl from src/tools into
src/backend/parser where it's actually used, and did some very minor
cleanup on the script.