Support passing environment variables to forked processes

[samskivert]

From my mailing list post:

I have "run" configured to fork off my Java process and I'd like to set
environment variables for the forked process. The code appears to
accommodate this (in Fork.scala):

val environment = builder.environment
for( (key, value) <- env )
environment.put(key, value)

but there's no way to get anything into 'env' (without writing my own code
to call into Fork), as ForkScala just passes Map.empty and ForkRun uses
ForkScala to actually fork and run my process.