Port over g_class_by_name subroutine from OneFS

Sponsored-by: EMC Isilon
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Make bsdinstall optional via WITH{OUT}_BSDINSTALL

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Merge branch 'master' of github.com:yaneurabeya/freebsd