Packfile cleanup: rename Parrot_readbc to Parrot_read_pbc, _loadbc => _load_pbc

[rurban]

Attached is a patch which:

- renames Parrot_readbc to Parrot_read_pbc (as heard on IRC)
- renames Parrot_loadbc to Parrot_load_pbc
- adds a third argument to Parrot_read_pbc which is stored in

the returned pf->options member, to be able to react in parrot and in other pbc utils differently.
Used for debugging pbc transformers.

- macrofy TRACE_PACKFILE printing (debug levels 1, 2 and 3)

Rationale: Debugging the pbc reader requires rebuilding libparrot and pdump, but this affects parrot also. parrot should not debug-print pbc transformers while testing.

Also included is the TT #258 TODO patch "pcb_disassemble options -[bho?], enable roundtrips". I can apply them seperately.