Use IPV6_MTU_DISCOVER for setting don't fragment on IPv6 sockets on Linux (was
using IP_MTU_DISCOVER, the IPv4 arg).
Updates #311
Signed-off-by: Val <valerie@tailscale.com>
Make the debugknob variable name for enabling peer path MTU discovery match the
env variable name.
Updates #311
Signed-off-by: Val <valerie@tailscale.com>