ifconfig(8): Sync with FreeBSD a bit and various cleanups

ifconfig(8): Sync with FreeBSD a bit and various cleanups

* Sync ifconfig.c a bit with FreeBSD; replace several warnx() with
  errx(), and improve error messages a bit.

* Remove unnecessary exit() after Perror().

* Sync if_clone.c with FreeBSD.

* Add inclusion guard to ifconfig.h.

* Make iseq() a static function for ifieee80211.c and regdomain.c,
  replacing the duplicate definitions of it; and use iseq()
  consistently to replace strncasecmp() and strcasecmp().

* Move "struct ident" from regdomain.c to regdomain.h, and add and use
  "enum IdentType".

* Various whitespace cleanups.

* Cleanup header file inclusion: remove duplicates, and reorder a bit.

* Fix various signed vs. unsigned comparisons.

* Fix some compilation warnings, and raise WARNS to 3.