Minor autoconf cleanups.
Fix mount table parsing stuff;.

Add a mnttab iteration API.
Fix a few thread function prototypes.
Cast curthread as kthread_t *.
Fix up the vdev_disk interface a little bit.  Needs more work.
Fix the ioctl interface.

Replacing components wholesale is NOT a good approach.  This is why we
were having problems with ioctl().  The ZFS_IOC_* constants did not
line up with zfs_ioc_vec[], so the wrong function pointer was getting
called.

Nuke the old ZFS_IOC_* constants and replace them with the enum, slightly
modified, that exists in ZoL.