Low: Core: Support PCMK_logpriority to set syslog priority

This request makes it possible to specify a syslog priority.

I am monitoring some INFO messages during production run in pacemaker-1.0. Therefore, I need those INFO messages similarly in 1.1.

The INFO messages are outputted only to _application log_ [1] in 1.1, so I checked the implementation of application log.

- [1] /etc/sysconfig/pacemaker

 ```
 export PCMK_debugfile=/var/log/ha-log
 ```

I figured out that 'copytruncate' is required in order to rotate the log using logrotate.

- eg. /etc/logrotate.d/ha

 ```
 /var/log/ha-log {
 missingok
 copytruncate
 }
 ```

Ummm... I can't use copytruncate because some logging data are lost.
Since I use syslog if the INFO messages are outputted, I wrote this patch.