numerous fixes for Linux IPv6 configuration using ifconfig/route/netstat

numerous fixes for Linux IPv6 configuration using ifconfig/route/netstat

Trying to ensure that none of the commands executed change in any way on the actual non-Linux OSes which
we cannot directly test with CI currently.

- add missing $route_syntax_gw
- need new syntax variants:
  - $ifconfig_syntax_add_inet6 ('add' on Linux, 'inet6' on *BSDs)
  - $route_syntax_ipv6 ('-6' on Linux, '-inet6' on *BSDs)
- special handling for IPv6 route addition deletion on Linux:
  - '-host'/-net' are REQUIRED on *BSDs but Linux 'route' rejects them
    ⇒ add $route_syntax_ipv6_{host,net} for these cases
  - Linux 'route' rejects local adapter's IPv6 address as gateway with "SIOCADDRT: invalid argument"
    ⇒ use 'dev $DEVICE' instead of 'gw $NETGW' in this case on Linux

Signed-off-by: Daniel Lenski <dlenski@gmail.com>