AIM logging function

Reviewer: @rlane

Introduce a logging function into aim_log_t, so that aim_log_t does not rely on aim_pvs_t internally,
Rework aim_pvs_* to provide logging functions for use with aim_log_t, if so desired.

As a side benefit, because aim_log_flag_t is passed into the logging function, the syslog priority is now set properly in aim_pvs_syslog.c.
Might want to return NULL if the logf isn't aim_pvs_logf.