[lldb] [Process] Add proper support for NetBSD core files with threads

Improve the support for processing NetBSD cores.  Fix reading process
identifier, thread information and associating the terminating signal
with the correct thread.

Includes test cases for single-threaded program receiving SIGSEGV,
and two dual-threaded programs: one where thread receives the signal,
and the other one when the whole process is signalled.

Differential Revision: https://reviews.llvm.org/D32149

llvm-svn: 355736