Allow setting home directory mode to something other then 0755

Its unusual (especially on multi user systems) to really want a home directory
thats 755 - it makes some sense to have 750 but 755 doesn't leave you much in
the way of privacy.

With this option the permissions can be changed per user with a default of 0755

I wasn't entirely convinced about the option name (homeperms) but 'mode' and 'home' both seemed a bit ambiguous.