* src/getargs.h (trace_e): Add trace_scan, and trace_parse.

* src/getargs.h (trace_e): Add trace_scan, and trace_parse.
* src/getargs.c (trace_types, trace_args): Adjust.
* src/reader.c (grammar_current_rule_prec_set)
(grammar_current_rule_dprec_set, grammar_current_rule_merge_set):
Standardize error messages.
And s/@prec/%prec/!
(reader): Use trace_flag to enable scanner/parser debugging,
instead of an adhoc scheme.
* src/scan-gram.l: Remove trailing debugging code.