if_ntb: MFV e26a5843: Move MW/DB management to if_ntb

This is the last e26a5843 patch. The general thrust of the rewrite was
to move more responsibility for Memory Window and Doorbell interrupt
management from the ntb_hw driver to if_ntb.

A number of APIs have been added, removed, or replaced. The old
DB callback mechanism has been excised. Instead, callers (if_ntb) are
responsible for configuring MWs and handling their interrupts more
directly.

This adds a tunable, hw.ntb.max_mw_size, allowing users to limit the
size of memory windows used by if_ntb (identical to the Linux modparam
of the same name).

Despite attempts to keep mechanical name changes to separate commits,
some have snuck in here. At least the driver should be much more
similar to the latest Linux one now -- making porting fixes easier.

Authored by: Allen Hubbe
Obtained from: Linux (Dual BSD/GPL driver)
Sponsored by: EMC / Isilon Storage Division