mtr-packet: Fall back to IPv4 only support if IPv6 sockets fail to open

mtr-packet: Fall back to IPv4 only support if IPv6 sockets fail to open

If we fail to open any IPv6 sockets, rather than aborting with an
unrecoverable error, fall back to IPv4 only support.  Socket
creation might fail, for example, when Linux is booted with
the kernel command-line "ipv6.disable=1".

In the case where opening IPv6 sockets fail,
'check-support feature ip-6' will indicate there is no support
for sending IPv6 probes.

Stricter error reporting revealed that test for protocols other
than ICMP were running on Cygwin.  Modified the tests such that
they won't run if the protocol isn't supported.