Move $LEIN_JVM_OPTS farther down (override all but cp and lein*)
Allow JVM file.encoding to be overridden by anything.

This allows profiles to set a -Dfile.encoding. Later, perhaps in 3.0,
JVM_OPTS should perhaps be moved farther down the list (along with
:jvm-opts?) -- but that could be a breaking change. (Environment is
sometimes set locally, sometimes set globally.)