Fix for issue 4151 (Default java heap not enough to launch import)

The idea is to improve what we currently have, which is relying on java ergonomics and only setting the '-client' and '-server' arguments as default java arguments.  As far as I can see with the new import code, the java ergonomics (this has been reproduced in Solaris and Mac OS X) are not enough to guarantee that the server will be able to make a small import (around 2000 entries) out of the box.

The proposed fix tries to set the following arguments to the server command-lines (start-ds and import-ldif in particular):

-Xms128m -Xmx256m

These arguments will be set if and only if:

They can be used while the setup is being run (and so the JVM supports them and the system where we are running is able to launch a JVM using them).

The ergonomics of the JVM where the setup is being run does not allocate a maximum heap that is higher than those values.  With this check we guarantee that we are not going to allocate less memory than what the JVM already does by default.