Let --trace have arguments.

Let --trace have arguments.
* src/getargs.h (enum trace_e): New.
* src/getargs.c (trace_args, trace_types, trace_argmatch): New.
(long_options, short_options): --trace/-T takes an optional
argument.
Change all the uses of trace_flag to reflect the new flags.
* tests/sets.at (Firsts, Nullable, Broken Closure): Use --trace=sets.
Strengthen `stage' portability.
* m4/stage.m4 (BISON_PREREQ_STAGE): New.
* configure.in: Use it.
Don't check for malloc.h and sys/times.h.
* src/system.h: Include them when appropriate.
* src/main.c (stage): Compile only when mallinfo, struct mallinfo,
times and struct tms are available.