Provide compatibility symlink for logical partitions:

1. Extend geom_dev by having it create the symlink (i.e. call make_dev_alias) based on the DIOCGPROVIDERALIAS ioctl. In this way the functionaility is generic and thus usable by any geom/provider.
2. Have g_part handle said ioctl through the devalias method, so that it's under control of the scheme itself. By design the alias will not be created for newly added partitions.