filemon exec: Cease tracing if credentials will change with the new image.

[bdrewery]

Summary

This also prevents tracing to a P_INEXEC process since it could race
with other processes attaching to it in filemon_event_process_exec() due
to the filemon_get_proc() race of incrementing ref and then locking the
filemon. With the no-P_INEXEC invariant in place the p_filemon may only
be the same or NULL when trying to drop it in
filemon_event_process_exec().

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division

Test Plan

Ran filemon with script(1) with a suid binary and confirmed that no tracing of
the suid binary or its children were traced. Upon returning from the suid
process, the parent process continued to show its own tracing information.

[bdrewery]

• Replace removed credential_changing with more proper credential_setid - only cease tracing if going setid