Fix several leaks of kernel stack data through paddings.

It is random collection of fixes for issues not yet corrected,
reported at https://tsyrklevi.ch/clang_analyzer/freebsd_013017/. Many
issues from that list were already corrected. Most of them are for
compat32, old compat32 or affect both primary host ABI and compat32.

The freebsd32_kldstat(), for instance, was already fixed by using
malloc(M_ZERO). Patch includes correction to report the supplied
version back, which is just pedantic.

Reviewed by: brooks, emaste (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D14868