clonefs improvements

- Mount /usr/src readonly and do not clone it in. This is a big improvement
  as the /usr/src tree has up to 48,000 files, and the rest of the tree
  is about 9,000 files.
- Don't clone /poudriere into the builder jails. This also avoids a ton of
  extra file copies that are not needed.
- Ignore /usr/src in mtree files now that it is read-only

Just building 3 ports (2 builders), this spedup the build from 259 seconds to 68 seconds