libs/core: improve luci.util.imatch() to not create temporary strings when iterating non-string values

libs/core: implement get_wandev(), get_wan6dev(), get_wannet() and get_wan6net() in luci.model.network

modules/admin-full: remove support for configuring alias interfaces, they're useless now that ifaces can be part of multiple networks

modules/admin-full: when deleting wifi-ifaces, remove all empty leftover networks, not only the first one

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>