Fix crash on compile/runtime kernel version mismatch

When unbound was built with linux-libc-headers-3.17 and run under kernel
3.14 it would fail with 'error: setsockopt(..., IP_MTU_DISCOVER,
IP_PMTUDISC_OMIT...) failed: Invalid argument'

Patch added to try IP_PMTUDISC_DONT when IP_PMTUDISC_OMIT fails.

https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=634