Make multibuild Holmake pass on reloc environment variable

Make parallel Holmake's output prettier

When a process fails, a chunk of its output up to the failure point is
output to the main screen. Also, a process that is killed because of a
general desire to give up (a failure elsewhere and no -k option, say),
now gets a red M-KILLED ("monitor killed") report.

Enable multiplexed builds for Poly/ML

Output is horrendous at the moment

Implement monitor cmds (e.g., kill) for Holmake

These are the commands that the client sends to the running worklist via
the callbacks. It even makes sense to allow for a GiveUpAndDie when the
client is generating a fresh job via the genjob callback.

Move ProcessMultiplexor to tools/Holmake/poly