When spawning subprocesses, fork() will fail if the memory of the parent
can't be duplicated in the child. This means large long-lived processes
will have problems using CodeJail repeatedly.

This introduces a long-lived proxy subprocess which then spawns the
actual subprocesses.  Because the proxy process starts small and stays
small, it will be able to spawn subprocesses even when the parent
process grows large.

The use of the proxy process is controlled by a "PROXY" limit, which
should be 0 or 1.  If neither is set, then the CODEJAIL_PROXY
environment variable determines whether the proxy is used.