Fix build on the recent current: wait_type WAIT_ANY intersects with the WAIT_ANY definition of sys/sys/wait.h
The r48589 promised to remove implicit inclusion of if_var.h soon. Prepare
to this event, adding if_var.h to files that do need it. Also, include
all includes that now are included due to implicit pollution via if_var.h

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Sponsored by: Nginx, Inc

MFC svn257176
Include necessary headers that now are available due to pollution
via if_var.h.

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Sponsored by: Nginx, Inc.

MFC svn257241