Include process IDs in core dumps.

When threads were added to the kernel, the pr_pid member of the
NT_PRSTATUS note was repurposed to store LWP IDs instead of process
IDs. However, the process ID was no longer recorded in core dumps.
This change adds a pr_pid field to prpsinfo (NT_PRSINFO). Rather than
bumping the prpsinfo version number, note parsers can use the note's
payload size to determine if pr_pid is present.

Reviewed by: kib, emaste (older version)
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D7117