Add host flag for relay

[kallydev]

Resolved #416.

Although net.Listen("tcp", addr) display IPv6, it also provides support for IPv4, ref golang/go#9334.

$ croc relay
[info]  2021/10/01 13:00:34 starting croc relay version v9.3.0-94b2aff
[info]  2021/10/01 13:00:34 starting TCP server on :9009
[info]  2021/10/01 13:00:34 starting TCP server on :9012
[info]  2021/10/01 13:00:34 starting TCP server on :9011
[info]  2021/10/01 13:00:34 starting TCP server on :9010
[info]  2021/10/01 13:00:34 starting TCP server on :9013

$ lsof -i :9009
COMMAND   PID     USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
main    31243 kallydev    5u  IPv6 0x786f7593799ac31b      0t0  TCP *:pichat (LISTEN)

$ croc relay --host 0.0.0.0
[info]  2021/10/01 12:58:49 starting croc relay version v9.3.0-94b2aff
[info]  2021/10/01 12:58:49 starting TCP server on 0.0.0.0:9009
[info]  2021/10/01 12:58:49 starting TCP server on 0.0.0.0:9011
[info]  2021/10/01 12:58:49 starting TCP server on 0.0.0.0:9013
[info]  2021/10/01 12:58:49 starting TCP server on 0.0.0.0:9012
[info]  2021/10/01 12:58:49 starting TCP server on 0.0.0.0:9010

$ lsof -i :9009
COMMAND   PID     USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
main    31367 kallydev    5u  IPv4 0x786f7593a7a196f3      0t0  TCP *:pichat (LISTEN)

$ croc relay --host :: 
[info]  2021/10/01 12:59:46 starting croc relay version v9.3.0-94b2aff
[info]  2021/10/01 12:59:46 starting TCP server on [::]:9009
[info]  2021/10/01 12:59:46 starting TCP server on [::]:9011
[info]  2021/10/01 12:59:46 starting TCP server on [::]:9010
[info]  2021/10/01 12:59:46 starting TCP server on [::]:9012
[info]  2021/10/01 12:59:46 starting TCP server on [::]:9013

$ lsof -i :9009
COMMAND   PID     USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
main    31308 kallydev    5u  IPv6 0x786f75937d8c79db      0t0  TCP *:pichat (LISTEN)

[schollz]

looks great!!! so cool, thanks