Refuse to try configuring IPv6 if MTU is <1280

IPv6 requires a minimum MTU of 1280.

If MTU is <1280, it appears that any-and-all IPv6-related configuration,
including setting addresses and routes, will immediately fail on
Linux, where iproute(8) gives very cryptic errors like:

    RTNETLINK answers: Invalid argument error

This will prevent vpn-slice from completing a working setup even for IPv4.

Rather than overlooking this problem or silently ignoring IPv6 configuration
issues, we should *fail* when IPv6 configuration is requested but the MTU is
too small, and request that the user add `--disable-ipv6` to the OpenConnect
command line, which should prevent OpenConnect from requesting or providing
any IPv6 configuration to vpn-slice.

Ping #148.

vpn_slice/__main__.py

@@ -455,6 +455,14 @@ def parse_env(environ=os.environ):
             print("WARNING: IPv6 split network (CISCO_IPV6_SPLIT_%s_%d_{ADDR,MASKLEN}) %s/%d has host bits set, replacing with %s" % (pfx, n, ad, nml, net), file=stderr)
         env['split' + pfx.lower()].append(net)
 
+    # IPv6 requires a minimum MTU of 1280.
+    # If the link is configured with a too-small MTU, it appears that any-and-all IPv6-related
+    # configuration, including setting addresses and routes, will immediately fail on Linux, where
+    # iproute(8) gives very cryptic errors like "RTNETLINK answers: Invalid argument error",
+    # preventing vpn-slice from completing a working setup even for IPv4.
+    if env.mtu < 1280 and env.myaddr6:
+        raise RuntimeError(f"MTU of {env.mtu} is too small for IPv6 (minimum 1280). Invoke OpenConnect with --disable-ipv6 to configure for IPv4 only.")
+
     return env
 
 # Parse command-line arguments and environment