Generate the known-dwarf.h macros

The previous known-dwarf.h was "stolen" from an elfutils build, but the
problem is that the constants within will depend on some minimum version
of dwarf.h.  A better solution is to steal elfutils' known-dwarf.awk
generating script instead, so the string tables will be generated from
the exact dwarf.h on the dwarvish build system.

This commit also adds dwstring.c to provide complete generated coverage
of all dwarf.h constant types.
Show attribute form and value

This supports addr, string, refs, data (just numeric), and flags.

Still TODO are blocks and exprloc, as well as more interpreted form data
like eu-readelf does, based on the context of a specific attribute code.
Store just a boxed Dwarf_Die in the tree

Having the original Dwarf_Die is easier than carrying the base Dwarf and
types bool around everywhere to get a Dwarf_Die from an offset.  This
also goes back to using a custom data function for cell renders, rather
than storing so much text in the tree all the time.
Minor fix for clang compatibility
Store just a boxed Dwarf_Attribute in its tree

Mirroring the change that was made for Dwarf_Die.