MFC r249558, r250145:

r249558:

Add a new set of notes to a process core dump to store procstat data.

The notes format is a header of sizeof(int), which stores the size of
the corresponding data structure to provide some versioning, and data
in the format as it is returned by a related sysctl call.

The userland tools (procstat(1)) will be taught to extract this data,
providing additional info for postmortem analysis.

PR:		kern/173723
Suggested by:	jhb
Discussed with:	jhb, kib
Reviewed by:	jhb (initial version), kib

r250145:

Introduce a constant, ELF_NOTE_ROUNDSIZE, which evidently declares our
intention to use 4-byte padding for elf notes.