HW keeps track of RX errors using several counters, each for
specific type of errors. Report RX errors to OS as sum
of all those counters: CRC errors, illegal bytes, checksum,
length, undersize, fragment, oversize and jabber errors.

There is no HW counter for frames with invalid L3/L4 checksums
so add a SW one.

Also add a "rx_errors" sysctl with a copy of netstat IERRORS
counter value to make it easier accessible from scripts.