Publish known-dwarf.h

Publish known-dwarf.h

- Make the macro names properly scoped (DWARF_ALL_KNOWN_* and
  DWARF_ONE_KNOWN_*).

- Simplify the calling convention: don't keep track of the comment
  associated with the enumerator, always use the non-_DESC
  DWARF_ONE_KNOWN_* callback.

- Install known-dwarf.h alongside libdw.h and others.

Signed-off-by: Petr Machata <pmachata@redhat.com>