trunks: Unify logging location

This CL defaults trunks daemon logging to syslog.
Logs can be forwarded to stderr by passing the --log_to_stderr switch.
trunks_client now logs to stderr by default.

Bug: 25567627
TEST=compile and run on DUT

Change-Id: I072a4c1dab9425f997faa0f68e8eae99f1b383fb