Defaulting the module on hid it from a host's config, leaving no single
place that lists what a host carries. Each host enables it explicitly
instead, at the cost of a step when adding one.
Default the module on rather than restating it per host, so a new host
commits without remembering the line. A host that should not carry a
personal identity sets enable to false.
Git identity lived only in one checkout's local configuration on one
machine, so it was invisible to every other checkout and lost on a
reimage. Declare it as a module instead, enabled on neogaia.
It is a module rather than base plumbing so a host that should not carry
a personal commit identity can decline it.
