tools/zep_dispatch: basic dual-stack support

[benpicco]

Contribution description

Murdock does not support IPv6 on link-local interfaces.
Therefore in order to being able to use ZEP dispatcher in automated tests on CI, we have to add dual-stack support.

Testing procedure

Make ZEP dispatcher listen on an IPv4 address

zep_dispatch 127.0.0.1 17754

Patch the gnrc_networking example to use IPv4 for ZEP

diff --git a/examples/gnrc_networking/Makefile b/examples/gnrc_networking/Makefile
index fcdcc3fb55..784d1fcaa8 100644
--- a/examples/gnrc_networking/Makefile
+++ b/examples/gnrc_networking/Makefile
@@ -47,7 +47,7 @@ USE_ZEP ?= 0
 # set the ZEP port for native
 ZEP_PORT_BASE ?= 17754
 ifeq (1,$(USE_ZEP))
-  TERMFLAGS += -z [::1]:$(ZEP_PORT_BASE)
+  TERMFLAGS += -z 127.0.0.1:$(ZEP_PORT_BASE)
   USEMODULE += socket_zep
 endif

And start two native instances with ZEP:

make -C examples/gnrc_networking USE_ZEP=1 all term

They are detected by the dispatcher

entering loop…
adding [0.0.0.0]:46934
adding [0.0.0.0]:60951

And can ping each other

main(): This is RIOT! (Version: 2022.01-devel-1563-g1b71cc-zep_dispatch-ipv4)
RIOT network stack example application
All up, running the shell now
> ping ff02::1
12 bytes from fe80::6cea:621a:16db:37bd%7: icmp_seq=0 ttl=64 rssi=0 dBm time=0.615 ms
12 bytes from fe80::6cea:621a:16db:37bd%7: icmp_seq=1 ttl=64 rssi=0 dBm time=0.581 ms
12 bytes from fe80::6cea:621a:16db:37bd%7: icmp_seq=2 ttl=64 rssi=0 dBm time=0.428 ms

--- ff02::1 PING statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.428/0.541/0.615 ms

Issues/PRs references

split off #17353

[aabadie]

The test procedure is working as expected and code changes look good.

ACK