Implement --trace=m4.

Implement --trace=m4.
* src/getargs.c (trace_types, trace_args): Accept trace_m4.
* src/output.c (output_skeleton): When set, pass -dV to m4.
Factor the handling of flags in m4.
* src/output.c (prepare): Rename the muscle names debug, defines,
error_verbose to debug_flag, defines_flag, error_verbose_flag.
* data/c.m4: Adjust.
(_b4_define_flag_if, b4_define_flag_if, b4_defines_if): New.
Use b4_define_flag_if to define other b4_FLAG_if macros.
(b4_location_if): As a consequence, rename as...
(b4_locations_if): this, for consistency.
Adjust all the skeletons.