Remove ability to write to struct if_data residing in struct ifnet
via net.link.generic.IFMIB_IFDATA.*.IFDATA_GENERAL sysctl. Reasons
for removal are:
- No code in tree uses this possibility.
- The documentation ifmib(4) doesn't say that such possibility
  exist. The example provided in manual page only reads data.
- On many interfaces the feature simply doesn't work, since they
  do accounting in hardware, and overwrite if_data on tick.

Sponsored by:	Nginx, Inc.
Use macros instead of referencing struct if_data that resides in ifnet.

Sponsored by:	Nginx, Inc.
o Remove struct if_data from struct ifnet. Now it is merely API structure
  for route(4) socket and ifmib(4) sysctl.
o Move fields from if_data to ifnet, but keep all statistic counters
  separate, since they should disappear later.
o Provide function if_data_copy() to fill if_data, utilize it in routing
  socket and ifmib handler.
o Provide overridable ifnet(9) method to fetch counters. If no provided,
  if_get_counters_compat() would be used, that returns old counters.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Move the restored #ifdef i386 test back inside the #ifdef _KERNEL block
where it originally was.