blackhole(4): disable for locally originated TCP/UDP packets

In most cases blackholing for locally originated packets is undesired,
leads to different kind of lags and delays. Provide sysctls to enforce
it, e.g. for debugging purposes.

Reviewed by: rrs
Differential revision: https://reviews.freebsd.org/D32718

(cherry picked from commit 3ea9a7cf7b09a355cde3a76824809402b99d0892)
(cherry picked from commit ad3ad06477d013371b95af673a9776c62f49a97f)