- Make the prophecy from 1997 happen and remove if_var.h inclusion
  from if.h.
- Remove unnecessary includes and declarations from if.h
- Remove unnecessary includes and declarations from if_var.h [1]
- Mark some declarations that are about to be removed in near
  future with comments, explaning why this declaration is still
  necessary.
- Protect eventhandler declarations with #ifdef SYS_EVENTHANDLER_H.

Obtained from:	bdeBSD [1]
Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.