Rename frontend keyword arrays to avoid conflict with backend.

ecpg and pg_dump each contain keyword arrays with structure similar
to the backend's keyword array.  Up to now, we actually named those
arrays the same as the backend's and relied on parser/keywords.h
to declare them.  This seems a tad too cute, though, and it breaks
now that we need to PGDLLIMPORT-decorate the backend symbols.
Rename to avoid the problem.  Per buildfarm.

(It strikes me that maybe we should get rid of the separate keywords.c
files altogether, and just define these arrays in the modules that use
them, but that's a rather more invasive change.)