- Const'ify the ahci_ids table.
- Use DEVMETHOD_END.
- Use NULL instead of 0 for pointers.

MFC after:	3 days
Correct naming of sysctl pmtud_blackhole_activated_min_mss.

Clarify some statements around PMTUD blackhole detection to make
the behavior more clear in the man page.

Submitted by:	Mikhail <mp@lenta.ru>
MFC after:	2 weeks