Gentoo init script

This is a relatively trivial change that adds a init.d script and a conf.d file for gentoo. In the Makefile, I renamed `install` to `install-exe`, added a new `install` that just depends on `install-exe` and `install-init`, and added `install-init` that detects Gentoo and installs the scripts to their respective locations.

The init script checks for various variables and adds any corresponding options.