Re-implement mapping_t::get_map()

which is needed only by some debug prints.
Handle .dSYM files properly.

Use the debug symbol pathname only and not a mix of the two.

When reading symbols from the debug symbol file for the current
executable, don't try to merge those sections into system mappings,
those are mappings on the wrong file.
Fix expected out for one of the tdumdstr tests
Add a tags: target.
Handle missing .dSYM/ debug symbol files cleanly
Always prepare the address index.

There's no point having this being a separate call that test programs
have to remember to do.
Fix describe_address() for the address index case

it was returning a NULL compile_unit_t* which was causing taddr2line to
crash, despite this being trivially calculable.
Add --enable-debug configure option

which uses the define _NP_DEBUG to enable various debug prints.