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.
This looks like it's more for debugging than for an end user. Maybe put this under boot verbose? The reason is that it's not an error until it's an error, meaning that if we're going to wait for the device when it's not there yet, then printing errors is confusing. Putting it under boot verbose is a nice way for people to get insight into why a boot may be stalled...