tweak the pflog ifname filter to include the terminating nul.

tweak the pflog ifname filter to include the terminating nul.

without this the code effectively just compares the start of the
string, which means you can get a prefix match unexpectedly. including
the nul in the comparison means it only works for whole interface
names, as it was originally intended.

reported by Aner Perez on misc@
ok deraadt@ (who also dug through the history behind this)