Add < list > command to query available interfaces

[jamofer]

Description

Add < list > command to NO_BUS mode:

client: < list >
server: < interfaces can0 can1 >

Lets clients discover buses without the UDP beacon (routed networks, -n). Purely additive, reuses the -i access list. Docs updated. (closes #77)

Test

With 1 interface

jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ ip -br link show type vcan
vcan0            UNKNOWN        <NOARP,UP,LOWER_UP> 
jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ ./build/src/socketcand -i vcan0 -l lo -p 1234 >/dev/null &
[1] 708039
jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ nc 127.0.0.1 1234
< hi >< list >
< interfaces vcan0 >< open vcan0 >
< ok >^C

With many interfaces

jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ ip -br link show type vcan
vcan0            UNKNOWN        <NOARP,UP,LOWER_UP> 
vcan1            UNKNOWN        <NOARP,UP,LOWER_UP> 
vcan2            UNKNOWN        <NOARP,UP,LOWER_UP> 
jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ ./build/src/socketcand -i vcan0,vcan1,vcan2 -l lo -p 1234 >/dev/null &
[1] 712932
jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ nc 127.0.0.1 1234
< hi >< list >  
< interfaces vcan0 vcan1 vcan2 >< open vcan0 >
< ok >^C
jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ nc 127.0.0.1 1234
< hi >< open vcan1 >
< ok >^C
jamo@CS-Jamo-Laptop:~/github/socketcand(list-interfaces)$ nc 127.0.0.1 1234
< hi >< open vcan2 >
< ok >^C