WIP: Implement backtrace

[kadler]

I've added some WIP code I had laying around to truly implement backtrace().

backtrace2.c is closer to backtrace.c, but doesn't compile. backtrace3.c was from my home directory, which does compile, but is from an older version before I cleaned up the comments and layout when I merged the placeholder backtrace.c

[abmusse]

Are we also going to implement backtrace_symbols_fd #3 ?

[kadler]

We can. It shouldn't be too difficult to make it work just by wrappering backtrace_symbols. The Linux man page says that backtrace_symbols_fd does not call malloc, which would require more work to implement. I did not see the same language in the Solaris, FreeBSD, or macOS man pages, though.

[ThePrez]

LGTM

[ThePrez]

@kadler , is this one ready to land?

[ThePrez]

Nevermind.... I was thinking we could land this one and pursue finishing this all up in a future thing, but that doesn't make a lot of sense

[kadler]

No, this is not ready to land. Not really intended to land either, more just to get visibility. Best for someone to take the code from this PR and fix it up in to a separate PR.