Remove ZFS_IOC_*_MINOR ioctl()s

Some overdue cleanup to bring the ZoL utilities back in sync with FreeBSD and Illumos in regards to zvol minor handling.  Because this patch adjusts the ZFS kernel/user ioctl() ABI we wanted to get this work done prior to 0.6.3 being tagged because it already contains the libzfs_core ABI changes.

The patch stack also includes some additional zvol related cleanup and man page updates.