When executing a module via external("*modname",..) with no extension

specified, if "filename.js" exists, it will be executed instead of
"filename.bin".
Previously, the ".bin" extension was always assumed if it wasn't specified on
the command-line. This modification allows exec/bullseye.js to take
precedence over exec/bullseye.bin without any change to the command-line
configuration. Execute "*filename.bin" if you want to specifically execute
"filename.bin" and not "filename.js" (if it exists).