Returns a unique nonexistent full path file name based on the name passed in.

Usage:  uniqfile <filename>

   ex:  uniqfile filename.ext
    ->   <cwd>\filename.ext   | 
    ->   <cwd>\filename.1.ext | ...
    ->   <cwd>\filename.2.ext | ...

   ex:  uniqfile %tmp%\mylogfile.log
    ->   c:\tmp\mylogfile.log   | 
    ->   c:\tmp\mylogfile.1.log | ...

Typical call sequence from a CMD script:
   set script_name=%~n0
   ...
   for /f %%i in ('uniqfile %tmp%\%script_name%.log') do set logfile=%%i
   call logmsg.cmd /t "Start %cmdline%"

NOTE: We write to the file, so other uniqfile calls will see it. Otherwise,
      multiple uniqfile calls will return the same unique file name.