Revision 237060

MFC r234188, r234463, r234465 (partially, except softclock_ih):

- Update the swi_add prototype after struct ithd was split up into
  struct intr_event and struct intr_thread. [1]
- Update the rest of struct ithd references.
- net_ih cookie has gone away.
- Document swi_remove(9).

PR:		docs/166864 [1]