journal: allow callers to specify OBJECT_PID=

When journald encounters a message with OBJECT_PID= set
coming from a priviledged process (UID==0), additional fields
will be added to the message:

OBJECT_UID=,
OBJECT_GID=,
OBJECT_COMM=,
OBJECT_EXE=,
OBJECT_CMDLINE=,
OBJECT_AUDIT_SESSION=,
OBJECT_AUDIT_LOGINUID=,
OBJECT_SYSTEMD_CGROUP=,
OBJECT_SYSTEMD_SESSION=,
OBJECT_SYSTEMD_OWNER_UID=,
OBJECT_SYSTEMD_UNIT= or OBJECT_SYSTEMD_USER_UNIT=.

This is for other logging daemons, like setroubleshoot, to be able to
augment their logs with data about the process.

https://bugzilla.redhat.com/show_bug.cgi?id=951627