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.

FWIW, I usually start julia directly under gdb. I think the only time I have used separate processes is when running julia under valgrind. (perhaps it is a matter of taste though?)