ifconfig: Support variable-argument commands

Add a new type of command, DEF_CMD_VARG, which takes an (argc, argv)
pair instead of a fixed number of arguments. This allows commands
to do their own argument parsing and accept a variable number of
arguments.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D51243

(cherry picked from commit 7d4a177efc653bc60a496ba0adf5cb4e0560fa07)