Move keywords.c/kwlookup.c into src/common/.

Now that we have src/common/ for code shared between frontend and backend,
we can get rid of (most of) the klugy ways that the keyword table and
keyword lookup code were formerly shared between different uses.
This is a first step towards a more general plan of getting rid of
special-purpose kluges for sharing code in src/bin/.

I chose to merge kwlookup.c back into keywords.c, as it once was, and
always has been so far as keywords.h is concerned.  We could have
kept them separate, but there is noplace that uses ScanKeywordLookup
without also wanting access to the backend's keyword list, so there
seems little point.

ecpg is still a bit weird, but at least now the trickiness is documented.

I think that the MSVC build script should require no adjustments beyond
what's done here ... but we'll soon find out.

src/include/parser/gramparse.h

@@ -4,8 +4,8 @@
  *		Shared definitions for the "raw" parser (flex and bison phases only)
  *
  * NOTE: this file is only meant to be included in the core parsing files,
- * ie, parser.c, gram.y, scan.l, and keywords.c.  Definitions that are needed
- * outside the core parser should be in parser.h.
+ * ie, parser.c, gram.y, scan.l, and src/common/keywords.c.
+ * Definitions that are needed outside the core parser should be in parser.h.
  *
  *
  * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group