[patch] $ACTIVEMQ_OPTS ignored in "activemq" startup script

[Brad Fritz <br...@fritzfam.com>]

I tried to submit this as a JIRA issue but wasn't able to login after
creating an account and receiving a confirmation.  :-/

Anyhow, there are two typos in assembly/src/release/bin/activemq that
prevent the default value for $ACTIVEMQ_OPTS from being overridden.

Th trivial patch attached adds the missing dollar sign and changes
"-n" (test for non-zero length) to "-z" (test for zero length) so that
$ACTIVEMQ_OPTS gets set to the default only if it's not already set.
Patch is against SVN trunk.

--Brad