iflib: Improve return values of interrupt handlers.

iflib was returning FILTER_HANDLED, in cases where FILTER_STRAY was more
correct. This potentially caused issues with shared legacy interrupts.

Driver filters returning FILTER_STRAY are now properly handled.

Submitted by: Augustin Cavalier <waddlesplash@gmail.com>
Reviewed by: marius, gallatin
Obtained from: Haiku (a84bb9, 4947d1)
MFC after: 1 week
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D19201