if $TMPDIR is defined set java.io.tmpdir=$TMPDIR.

Java ignores the $TMPDIR variable. This surprises
users who expect the Go server and agent to use $TMPDIR
as the temporary directory.

Fixes #223.
Make SERVER_STARTUP_ARGS readable.
Merge pull request #267 from sahilm/tmpdir

if $TMPDIR is defined set java.io.tmpdir=$TMPDIR.