Fix 'Mounting packages from' being blank when ATOMIC_PACKAGE_REPOSITORY is disabled

Reported by:	marino
Clarify what atomic package repository does
Fix spelling of optionally

Submitted by:	marino
Simplify the PACKAGES_ROOT code

Submitted by:	marino
Sync with trunk, update ATOMIC_PACKAGE_REPOSITORY description to
indicate true default value (off)
Poudriere can't unmount /var/db/ports, change to /options

Apparently putting a null mount on a tmpfs mount on top of another
null mount confuses DragonFly, doing umount on /var/db/ports results
in "Invalid Argument".  Just set it to /options, which is cleaner
anyway (imo).