Add if_inc_counter() and if_get_counter_default() functions that do
access to ifnet counters for code compatibility with FreeBSD 11.

This is direct commit to stable/10.

Discussed with:	glebius@, arch@
MFC: r272490 (partial)

  - MFi386: Add compile-with option for tau32-ddk.c.
  - Whitespace change.
MFC: r272491

  Reduce diffs against i386.