Mark missing PID cache data separately from flags.
Mark missing PID cache data separately from flags.

This simplifies the syslog() path a little, and allows us to express
"PID should be cached but no value is set.".

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

deirf/libumberlog