When reducing initial empty rules, Bison parser read an initial

When reducing initial empty rules, Bison parser read an initial
location that is not defined.  This results in garbage, and that
affects Bison's own parser.  Therefore we need (i) to extend Bison
to support a means to initialize this location, and (ii) to use
this CVS Bison to fix CVS Bison's parser.
* src/reader.h, reader.c (epilogue_augment): Remove, replace
with...
* src/muscle_tab.h, src/muscle_tab.c (muscle_code_grow): this.
* src/parse-gram.y: Adjust.
(%initial-action): New.
(%error-verbose): Since we require CVS Bison, there is no reason
not to use it.
* src/scan-gram.l: Adjust.
* src/Makefile.am (YACC): New, to make sure we use our own parser.
* data/yacc.c (yyparse): Use b4_initial_action.