Few more minor if_vmx tweaks

 - Allow the Rx/Tx queue sizes to be configured by tunables
 - Bail out earlier if the Tx queue unlikely has enough free
   descriptors to hold the frame
 - Cleanup some of the offloading capabilities handling
Reduce WARNS to 0 for dig, host, and nslookup to make them
compile with the optional WITH_BIND_SIGCHASE.

Submitted by:	Andre Albsmeier <Andre.Albsmeier@siemens.com>
Approved by:	delphij (mentor, implicit)
MFC after:	3 days
Sponsored by:	DK Hostmaster A/S