Bug 871475 - 2/6: fix +CGDCONT/+CGACT

Three things to fix here:

1. From 3GPP TS 27.007 subclause 10.1.10 "PDP context activate or
deactivate +CGACT", "The read command returns the current activation
states for all the defined PDP contexts." but now we return only
active ones.

2. From 3GPP TS 27.007 subclause 10.1.1 "Define PDP context
+CGDCONT", "(about <PDP_addr>) The read form of the command will
continue to return the null string even if an address has been
allocated during the PDP startup procedure." So we should return the
given <PDP_addr> in the response of the read form.

3. Return correct supported cid range <1-MAX_DATA_CONTEXTS> in the
test form of +CGDCONT.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 3/6: intantiate additional rmnet nics.

Create <MAX_NET_CLIENTS> additional NIC instances when radio is
enabled. These NICs will have the name "rmnet<N>".

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 4/6: support link down for SMC91C111

Original SMC91C111 doesn't support link up/down reporting (EPHSR).
This patch implement it and one may call |do_set_link| to change the
link status.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 5/6: link up/down in AT+CGACT handler.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 6/6: implement AT+CGCONTRDP.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>