EDNS OPT writing

Fixes a problem, where OPT RR was not included into the packet, when a previous RRs didn't fit.

The changes seem ok, I've added:

• reservation size calculated from query, not from prepared RR
• opt not included in error responses
• check for opt/tsig section (#286 (closed)) Somebody else should recheck this. @jkadlec

Changes by @mvavrusa look good.