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

src/interfaces/ecpg/preproc/ecpg.c

@@ -9,7 +9,7 @@
 
 #include "getopt_long.h"
 
-#include "extern.h"
+#include "preproc_extern.h"
 
 int			ret_value = 0;
 bool		autocommit = false,