setup, pulseaudio, git: Add PulseAudio configuration; substituted dotfiles.

Add a notion of a `substituted dotfile', which is installed by applying
a simple sed(1) script to a skeleton rather than messing with symbolic
links.

Install some basic PulseAudio configuration.  Pulse needs this hacking
because it doesn't let us refer to our home directory without an absolute
pathname, which is a total nuisance.

Adopt .gitconfig under this scheme.  The source moves to dot/gitconfig.in
now.