em_rxeof is only called from em_intr, and only if IFF_RUNNING is
set. em_rxeof doesnt have to check that flag again.
in em_rxeof, when the ifp stack var is declared its initted to the
right value out of the softc. then its assigned the same value again
after the rest of the var decls.

well, it used to be. not after this commit.
Document that shm_open() et al have been available since OpenBSD 5.4