Implement Logfile name for the Hard Status Line

This allows the active log file name to be displayed in the status.

Using the following configuration, Screen can be used to log
data from serial lines, with quick access to the log file by
reference of the status line.

Example .screenrc:

hardstatus string '%{= kG}[ %{G}%H %{g}][%N][ %=
%{=kw}%?%-Lw%?%{r}(%{W}%n*%f%t%?(%u)%?%{r})%{w}%?%+Lw%?%?%=
%{g}][%{B}%Y-%m-%d %{W}%c %{g}]'

deflog on
logfile /home/kbingham/logs/%Y-%m-%d_%0c.%t.log