Fix problem with reading runMcV-Prefs.bat.

This removes the (windows-specific) “SET “ prefix *after* splitting
each “SET OPTION=VALUE” value line on the first occurrence of the
equals character.

[1534]
Improve handling of JVM_OPTIONS startup pref.

Prior to this commit both the GC options and whatever the user
specified were being stuffed into the JVM_ARGS variable. Now the
respective values are put into “GC_ARGS” and “JVM_ARGS”.

[1534]