Document all sorts of C debugging tips

Fulfills a pledge I made on on julia-dev. This moves some info I wrote from the FAQ to the "developer docs", and blatantly steals from sources posted by @ihnorton and @staticfloat. Finally, there's a small number of whitespace changes.

What's funny is that I'd swear Ctrl-C did not interrupt to `gdb` when I first tried it oh-so-long ago. So that's why I figured this out.

I'll change this (probably just delete the whole section).