ntb_hw(4): Allow any x86 PAT caching flags for MW defaults

Replace the hw.ntb.enable_writecombine tunable with
hw.ntb.default_mw_pat. It can be set with several specific numerical
values to select a caching type. Any bogus value is treated as
Uncacheable (UC).

The ntb_mw_set_wc() KPI has removed the restriction that the selected
mode must be one of UC, WC, or WB.

Sponsored by: EMC / Isilon Storage Division