SI-4907 scala.bat now honors -J options on Windows.

This makes scala.bat parse and use -J options like the sh
version.
Specifically:
 - if %JAVA_OPTS% is not defined and no -J option is given, it
   uses the default values embedded in the script like before,
 - if %JAVA_OPTS% is not defined but there are -J options, they
   are appended to the default values and are also given as
   scala arguments, as in:

   scala -J-Xmx512m Test

   will result in:

 java -Xmx256M -Xms32M -Xmx512m [...] [scala main] -J-Xmx512m Test

 - if both %JAVA_OPTS% and -J options are defined, the latter are
   appended to the former, as in:

   set JAVA_OPTS=-Xmx512m
   scala -J-Xms64m Test

   will result in:

   java -Xmx512m -Xms64m [...] [scala main] -J-Xms64m Test

This seems to be the exact same behavior as the sh launcher.