cleaned up signal handler and improved stack trace generation

[firewave]

See #5540 (review) for related discussion.

[firewave]

This needs actual unit tests for the stack trace code. I have been meaning to do this for quite a while since they different in format between Windows and Linux. Also to leverage them in daca instead of calling gdb after a crash (see https://trac.cppcheck.net/ticket/11255).

[firewave]

This omits all frames above our own code now without adjusting the frame number:

$ test-signalhandler abort
Internal error: cppcheck received signal SIGABRT - abort or assertion
Callstack:
#4  0x7f57279ff50e in my_segv()
#5  0x7f57279ff5ac in cmake.output.signal/bin/test-signalhandler(main+0x70)
#6  0x7f57274676ca in /lib/x86_64-linux-gnu/libc.so.6(+0x276ca)

Please report this to the cppcheck developers!
Aborted (core dumped)

I guess I will make this behavior optional in case the code needs to show all frames in the future.