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 behavior of opening two LOG
sessions with the same label is undefined.

The only configuration and policy available is the option to truncate
files at the start of the LOG session, which is disabled by default.

: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="" truncate="yes"/>
!   </config>
! </start>