Update dwarf branch name, license and contributor policy.

Update dwarf branch name, license and contributor policy.

* Change name from "Red Hat elfutils" to "elfutils".
* Update license of standalone tools and test from GPLv2 to GPLv3+.
* Change license of libraries from GPLv2+exception to GPLv2/LGPLv3+.
* Add Developer Certificate of Origin based contributor policy.

dwarflint:

- All files updated to GPLv3+.

libdw/c++:

- All files updated to GPLv2/LGPLv3+.

src:

- All files updated to GPLv3+. Except dwarf-opcodes.h, dwarfstrings.c
  and dwarfstrings.h, which were meant as library files and have been
  updated to GPLv2/LGPLv3+.

tests:

- All files updated to GPLv3+. Except for the test bz2 data files.

Signed-off-by: Richard Fontana <rfontana@redhat.com>
Signed-off-by: Mark Wielaard <mjw@redhat.com>