Allow a path to be specified for the chansrv log file

[matt335672]

Forward port of #3344

Fixes #3337

A parameter LogFilePath is added to the [ChansrvLogging] section in sesman.ini. Manpage excerpt:-

       LogFilePath=string
              Directory  for  storing  the chansrv log file. This setting only
              applies to chansrv. The sesman log file  is  always  created  in
              /var/log.

              Created  if  it doesn't exist.  If first character is not a '/',
              this is relative to $HOME, where chansrv is normally started.

              The following substitutions are made in this string:-
                  %U - Username
                  %u - Numeric UID
                  %% - Percent character

              This is most useful if you are using NFS-mounted  home  directo‐
              ries, and wish to move the chansrv log file to the local disk.

              If this parameter isn't specified, the log file is stored in one
              of the following locations :-
                  -   $CHANSRV_LOG_PATH
                  -   $XDG_DATA_HOME/xrdp
                  -   $HOME/.local/share/xrdp