Holmake: key monitor function on combination of PID and tag-string

The latter was probably unique when the tags were the filenames that
were either from the current directory, or had paths attached. But
if the printing form is the tag, which means the path is always
stripped, multiple files of the same name may be being built in
parallel. Then they can be distinguished by the PID of the process
that is building them.

Make sure hollog files are stored under right directory

Also normalise theory log files to always name without the
.dat/sig/sml suffix.

Closes #607 (as a side effect).

With test.

Progress: now some recursive building happens occasionally

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