net: shell: conn: Dump local/remote ports for contexts.

Without port numbers, information printed by net shell's "conn" command
is unclear and non-differentiating. Also, improve handling of unknown
address families - don't abort early, still pring context address, iface
address, etc.

Example of "conn" output with these changes from echo_server sample:

net> conn
     Context   	Iface         Flags Local           	Remote
[ 1] 0x001126e0	0x001121a0    6DU   [::]:4242	[::]:0
[ 2] 0x00112738	0x001121a0    4DU   0.0.0.0:4242	0.0.0.0:0
[ 3] 0x00112790	0x001121a0    6ST   [::]:4242	[::]:0
[ 4] 0x001127e8	0x001121a0    4ST   0.0.0.0:4242	0.0.0.0:0

TCP        Src port  Dst port   Send-Seq   Send-Ack  MSS    State
0x00112b00     4242         0  497073352          0  1280
0x00112b98     4242         0  497543793          0  1460

Change-Id: I2f03efd933979b5571f608c28a7a7a4bd7f5346a
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>