Added btp_elf_find_fde_for_start_address for callgraph building.
libunwind: Unwind all threads of a coredump.

Fixes #13.
Use btp_elf_find_fde_for_start_address for callgraph building.
Survive broken coredumps and incorrect unwinding process.
Merge branch 'master' of github.com:karelklic/satyr