- Disable hardware checksumming until it is properly tested.
- Don't discard frames if the dropped or error flag is set.
- Don't remove the last 4-bytes of every packet.
- Add extra range check for data position offset when receiving data.

MFC after:	1 day
PR:		191432
Micro-manage clang to get the expected inlining for cpu_search().
Mark cpu_search_lowest/cpu_search_highest/cpu_search_both as noinline,
while cpu_search() gets always_inline.  With the attributes set,
cpu_search() is inlined in wrappers, and if()s with constant
conditionals are optimized.

On some tests on many-core machine, the hwpmc reported samples for
cpu_search*() are reduced from 25% total to 9%.

Submitted by:	"Rang, Anton" <anton.rang@isilon.com>
MFC after:	1 week
Note that most errors are possible for all syscalls from utimes(2)
family.  Minor wording corrections.

Based on the suggestions by bde.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week