Minor cleanup before fixing Mantis bug 351. (This is NOT the bug fix.)

compiler/hlds_pred.m:
    Replace some uses of booleans in proc_infos with specialized types.
    Put the fields involved into a more logical order. Give their access
    predicates more meaningful names.

    When creating new proc_infos, give names to all the arguments of
    proc_infos and proc_sub_infos. (I believe Mantis bug 351 is in
    these arguments.)

compiler/hlds_module.m:
    Use the new types in hlds_pred.m where relevant.

compiler/parallel_to_plain_conj.m:
    Add some conditionally combined debugging output that helped me track down
    the problem.

compiler/loop_inv.m:
mdbcomp/mdbcomp.goal_path.m:
    Minor cleanup of some comments.

compiler/code_info.m:
compiler/deforest.m:
compiler/dep_par_conj.m:
compiler/distance_granularity.m:
compiler/granularity.m:
compiler/hlds_out_pred.m:
compiler/inlining.m:
compiler/introduce_parallelism.m:
compiler/lambda.m:
compiler/mark_tail_calls.m:
compiler/mercury_compile_middle_passes.m:
compiler/par_loop_control.m:
compiler/proc_gen.m:
compiler/simplify_goal_call.m:
compiler/simplify_goal_conj.m:
compiler/simplify_info.m:
compiler/simplify_proc.m:
compiler/trace_gen.m:
compiler/trace_params.m:
    Conform to the changes above.
Avoid some C compiler warnings in the trace directory.

trace/mercury_trace.h:
    Avoid a warning from the C compiler about casting -1 to an enum
    by adding bool next to the enum that says whether the enum is actually
    defined, or is supposed to be set later (the latter is what the cast -1
    meant).

trace/mercury_trace_cmds.h:
    Rename the type MR_Trace_Command_Info to MR_TraceCmdTableEntry,
    since this (a) says more about the values of the type, (b) avoids
    confusion with the separate MR_TraceCmdInfo type (the one that
    contained the enum mentioned above), and (c) avoids combining
    underscores with CamelCase.

trace/mercury_trace_spy.c:
    Avoid some warnings about switches on enums specifying -1 as a case.

trace/mercury_trace.c:
trace/mercury_trace_*.[ch]:
    Conform to the above changes.