Fix core corruption caused by race in note_procstat_vmmap

This fix is spiritually similar to r287442 and was discovered thanks to
the KASSERT added in that revision.

NT_PROCSTAT_VMMAP output length, when packing kinfo structs, is tied to
the length of filenames corresponding to vnodes in the process' vm map
via vn_fullpath. As vnodes may move during coredump, this is racy.

We do not remove the race, only prevent it from causing coredump
corruption.

• Add a sysctl, kern.coredump_pack_vmmapinfo, to allow users to disable kinfo packing for PROCSTAT_VMMAP notes. This avoids VMMAP corruption and truncation, even if names change, at the cost of up to PATH_MAX bytes per mapped object. The new sysctl is documented in core.5.

• Fix note_procstat_vmmap to self-limit in the second pass. This addresses corruption, at the cost of sometimes producing a truncated result.

• Fix PROCSTAT_VMMAP consumers libutil (and libprocstat, via copy-paste) to grok the new zero padding.

Reported by: pho (https://people.freebsd.org/~pho/stress/log/datamove4-2.txt)
Relnotes: yes
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3824