Allow LEIN_JVM_OPTS to override -Dfile.encoding.

This allows the JVM to be run with an encoding other than UTF-8; my
own use-case for this is running tests to check if I'm forgetting to
specify UTF-8 in any character encoding/decoding calls.
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.)
Merge pull request #1676 from timmc/alt-jvm-encoding

Allow $LEIN_JVM_OPTS to override -Dfile.encoding