Look for IWAD if -FILE but no -IWAD on cmdline

Matching the new functionality of the WAD launcher, if a PWAD is
specified on the command-line with -FILE, but no IWAD is specified with
-IWAD, look for the required IWAD automatically in the same folder as
the PWAD. If that fails, check the "wadfolder" setting in doomretro.cfg.