Alter the qname bpf filter to make it const.

Alter the qname bpf filter to make it const.

This program needs to use a runtime value (a file descriptor), so in its
current form it is a non-const HUGE initializer, and causes the compiler
to emit a lot of code to initialize the local variable with the program
data, eating a lot of memory while there.

By changing the program to use a value of zero for the file descriptor,
the initializer becomes const; we need however to search for that
instruction and pach it before using the program, and this is not the
code I am most proud of... but when life gives you lemons, etc.

Signed-off-by: Miod Vallat <miod.vallat@powerdns.com>