Simplify hang detection by stealing the techniques used in ixl(4) and
applying them to em(4).

Rely on iterations through the local timer, and the tx queue state to
determine if an actual hang has occurred. Any time a descriptor is used
(packet sent), the tx queue is flagged as busy. Then when txeof runs, it
either clears the flag when all is clean, or resets it to 1 if ANY are
cleaned, if nothing is cleaned it increments the flag.

Local timer simply checks to see if busy ever reaches MAX (10, which
is compile time configurable), and then sets it as HUNG, at that point
there is one more timer cycle in which to have any cleans, if not a
watchdog reset will occur.

Differential Revision: https://reviews.freebsd.org/D2019
Submitted by: jfv
Reviewed by: hiren
Obtained from: Intel Corporation
MFC after: 2 weeks
Relnotes: Yes
Sponsored by: Limelight Networks