Added --enable-precheck-nonchroot, '?b' extended directive (see MANUAL)  

Signed-off-by: nixnodes <code@eth.si>
Made paths relative to siteroot

Signed-off-by: nixnodes <code@eth.si>
Signed-off-by: nixnodes <code@eth.si>
--nobuffer broken (rev. 8)

Signed-off-by: nixnodes <code@eth.si>