lib/home.c: Introduce functions for building pathmames in home directories.

lib/home.c: Introduce functions for building pathmames in home directories.

Split the home-directory logic of `config_userconf' into its own file.
Arrange to only calculate the home directory once.

Most invasively, use this new functionality in place of looking up
`$HOME' and building pathnames by hand.

This change doesn't add much functionality, but it does add a little.
Most notably,

	$ unset HOME; disobedience

doesn't segfault any more.  More subtly, the logic for finding one's
home directory is now consistent.  On Windows, we still use `%APPDATA%';
on Unix, we now /always/ use `$HOME' first, and then fall back to
`getpwuid(getuid)->pw_dir' if that's unset.  Previously, the
configuration reader would ignore `$HOME' and use the password database,
while Disobedience used `$HOME' exclusively and crashed if it was
unset.  This is all obviously silly.