However, the in-tree code is still broken in that too many interfaces
  causes it to ignore some of them.  For example:

          dhcpd=YES   dhcpd_flags="-q vlan12 vlan13 vlan14 vlan15"


  This will not work on my machine.  It claims it cannot find vlan15.

(I presume you are using current, since 4.0 hasn't changed SIOCGIFCONF
to have sockaddr_storage.)  I just committed a fix that I can't easily
test.  Please update to 1.11 of src/dist/dhcp/common/discover.c and let
me know if that fixes your problem.