corekeeper: Always include PID in core dump names.

corekeeper: Always include PID in core dump names.

Some distributions automatically set /proc/sys/kernel/core_uses_pid to 1
and others leave it at its default setting of 0.  That means that, with the
core_pattern that corekeeper was setting, on the former distributions the
PID would be included in core names and on the latter the PID would be
omitted.  For consistency, this commit forces the PID to be in the core
file name in either case (note that putting %p in core_pattern causes
the core_uses_pid setting to be disregarded).

CC: Martin Casado <casado@nicira.com>