ifconfig: split argument parsing and actual execution logic

ifconfig: split argument parsing and actual execution logic

Reduce the amount of global variables by creating the dedicated
 ifconfig_args structure and use it as a context-passing variable.
Simplify the code by moving all argument preparation code a
 separate function.

Reviewed by: kp (previous version)
Differential Revision: https://reviews.freebsd.org/D39932
MFC after:	2 weeks