LOG server that writes log messages onto a file system.

Log files are creating in a directory tree formed from session labels.
As an example the session label "init -> nitpicker" would create
a log file at "init/nitpicker.log".

The option to truncate files at the start of each LOG session is available
through session policy, as well the option to merge the logs of any
session matching a given policy. When a merged policy label contains a
trailing "->", the log filename takes the name of the next label element.

:Example configuration:
! <start name="log_file">
!   <resource name="RAM" quantum="1M"/>
!   <provides><service name="LOG"/></provides>
!   <config>
!     <policy label="nic_drv" truncate="no"/>
!     <policy label_prefix="cli_monitor -> " merge="yes"/>
!     <policy truncate="yes"/>
!   </config>
! </start>