Feature: Add new PCP command to trigger log rotation

Feature: Add new PCP command to trigger log rotation

Currently the only way to trigger log rotation in logging collector process
is to send SIGUSR1 signal directly to logging collector process.
However, I think it would be nice to have a better way to do it with an external
tool (e.g. logrotate) without requiring knowledge of the logging collector's PID.

This commit adds a new PCP command "pcp_log_rotate" for triggering log rotation.