Allow for CPU-specific kernel to be set from `zone_options_t`

[k0ekk0ek]

At some point, NSD will use a more efficient database. The prototype checks what "kernel" to use repeatedly, for parsing zone files, that will also be the case (i.e. once per invocation of zone_parse). An application should select the "kernel" to use at startup for consistency.

[k0ekk0ek]

Also include the option to set a preferred kernel (e.g. westmere, fallback), so we can include an option in NSD to "forcefully" set a specific kernel. Useful for situations where a hypervisor returns bad data for the CPUID instruction (not the case in #222, but would be useful to work around the bug from a user perspective). Also be sure to allow for convenient fetching of the associated kernel name so we can print in the NSD logs which kernel has been selected (see NLnetLabs/nsd#354).