Rename ecpg's various "extern.h" files to have distinct names.

Rename ecpg's various "extern.h" files to have distinct names.

This should reduce confusion, and in particular make it safe to
copy typename.c into preproc/ and compile it there.

This doesn't affect anything outside ecpg, and particularly not
end users, because these files don't get installed; they just
exist to share declarations among the .c files of each subdirectory.

Discussion: https://postgr.es/m/31364.1543511708@sss.pgh.pa.us