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) and bumps the
version of prpsinfo to 2.
Note: We could perhaps choose to leave the version at 1 and require
core dump parsers to use the size of the note data to determine if
pr_pid is present or not. I'm open to whatever folks think is best.