Restructure how we look for Read files slightly.

Restructure how we look for Read files slightly.

When searching for files with the Read command, check to see if the file
first of all has an absolute path and if so open it.  If it's relative
then search for it in either FVWM_USERDIR or FVWM_DATADIR as before.  If
the file still can't be found, try and open it outright.  This will
allow therefore:

../../some_file

as a valid filename -- of course this really only makes sense when
invoking fvwm with "-f" on the command-line, as relative paths to config
files in this way should be built up using "$."