Derive Emacs nix-mode from prog-mode.

Emacs 24.1 introduced the notion of "basic major modes" and among these
is prog-mode, see section "23.2.5 Basic Major Modes" in the Emacs
manual. The prog-mode basic major mode is recommended as a base for
derived major modes that are intended for editing source code.
Settings: Add bool get()
Add some instrumentation for debugging GC leaks
Add Make flag to disable optimization
On Linux, disable address space randomization