Display information about faulting thread from core dumps

[basil]

Core dumps have a NT_SIGINFO section that we could use in pstack(1) to display information about the faulting thread:

Field	Maps to
si_signo	Signal number
si_code	Signal code (segfault reason etc.)
si_errno	Associated errno
union fields	Faulting address (for SIGSEGV), sender PID (for kill), etc.