luci-mod-rpc: more auth/login fixes, expose further libraries

luci-mod-rpc: more auth/login fixes, expose further libraries

The previous attempt to fix authentication broke login functionality so
rework the code once again, this time with referencing helper functions
directly via the controller scope.

Furthermore, properly expose luci.sys.wifi.getiwinfo() and luci.ip.

For getiwinfo(), the RPC wrapped function accepts one further optional
parameter specifying the operation to invoke on the iwinfo instance.
If no operation is specified, a summary object containing all info
without country and scan list is returned.

Example to obtain iwinfo summary object:
  curl --cookie sysauth=... \
       --data '{"method": "wifi.getiwinfo", "params": ["wlan0"]}' \
		"http://192.168.1.1/cgi-bin/luci/rpc/sys"

Example to obtain iwinfo scan list:
  curl --cookie sysauth=... \
       --data '{"method": "wifi.getiwinfo", "params": ["wlan0", "scanlist"]}' \
		"http://192.168.1.1/cgi-bin/luci/rpc/sys"

The exposed luci.ip class uses a similar approach to allow invoking
instance methods on cidr objects. The new(), IPv4(), IPv6() and MAC()
constructors accept two further optional arguments, with the first
specifying the instance method to invoke and the second the value to
pass to the instance method.

Example to get list of IPv4 neighbours (ARP entries):
  curl --cookie sysauth=... \
       --data '{"method": "neighbors", "params": [{"family": 4}]}' \
		"http://192.168.1.1/cgi-bin/luci/rpc/ip"

Example to add 100 hosts to a network address:
  curl --cookie sysauth=... \
       --data '{"method": "IPv4", "params": ["192.168.0.1", "255.255.255.0", "add", 1000]}' \
		"http://192.168.1.1/cgi-bin/luci/rpc/ip"

Signed-off-by: Jo-Philipp Wich <jo@mein.io>