Enable ServerPack XML to specify recommended system minimums

[smbarbour]

Memory settings primarily. Will not be a hard restriction, but the user should be warned if their settings are not sufficient.

For memory, it needs to be able to know that 1024M = 1G

[allaryin]

Min memory and PermGen are the only two attributes that work here.

[Baughn]

Pack instructions for my server currently say "Make sure you use Java 8, and oh, go into the MCU settings and set maximum memory to at least 6G".

It would be great if neither of those were needed. Well, you can't install Java 8, but you can check if it's there.

[allaryin]

This issue probably also depends on per-instance memory settings in the gui. Currently, MCU-Swing only supports a single global set of JVM arguments.