homedir: s/dotdir/overlaydir/g, and fix a few enable/disable bugs
homedir: s/dotdir/overlaydir/g, and fix a few enable/disable bugs

"Overlay" is more general purpose than just "dot"files, so don't mangle
names (bonus) and be more flexible by allowing arbitrary links into ~
(double bonus).

Also:
- If a package is already enabled, don't try to enable again
- Fix more bugs in the link calculation

josh-berry/homectl,josh-berry/homectl