mcmodln - the mod linker

Rough version of what it will look like once finished.  It does work,
though.

Some structure requirements:

/
+ <clients directory>
| ` <client instance>
|   ` VERSION
` mods
  ` <mod name>
    + ACTIVATE
    ` <MC version>
      ` <versions>

In ``ACTIVATE'' goes the line describing how to install the mod.  It
currently either is "#mod" or "#coremod".

In ``VERSION'' goes the version string for Minecraft that describes
the current instance.  Only mods that work in VERSION can be linked
this way.

For ease of use, a mod version "LATEST" is defined as the default.  To
use this for a mod, create a symlink ``LATEST''.