mxmlc shell script update

Hey, first time playing around with flex i stumbled over a hickup in the mxmlc shell script undex linux.

I could not symlink to the mxmlc script to simplify setting up multiple versions/environment.

The flex SDK I am using is 3.4.1 build 10084.

I modified the shell script to lookup the java binary using JAVA_HOME instead of relying on PATH, and setup the automatic path resolution to resolve absolute url's using readlink.

I also decided to use exec to make sure that file descriptors and signals where directly passed to the executed java process.

I am sure that it isn't perfect, there might be some issues about finding JAVA_BIN under cygwin, but I sure would like to se symlinks resolved properly upstream.

Hope this is useful!