Plug some networking sysctl leaks.

Various network protocol sysctl handlers were not zero-filling their
output buffers and thus would export uninitialized stack memory to
userland. Fix a number of such handlers.

Reported by: Thomas Barabosch, Fraunhofer FKIE
Reviewed by: tuexen
MFC after: 3 days
Security: kernel memory disclosure
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18301