Separate the key word list that lived in keywords.c into a new header file

Separate the key word list that lived in keywords.c into a new header file
kwlist.h, to avoid having to link the backend object file into other programs
like pg_dump.  We can now simply symlink a single source file from the backend
(kwlookup.c, containing the shared routine ScanKeywordLookup) and compile it
locally, which is a lot cleaner.