Give buildheap+Holmake --mt c/line opt for control of thread count

Fix Holmake's -I flag so it is not applied recursively

The -I flag to Holmake was being re-interpreted with each recursive
entry of a fresh directory, so that when the included directory was
entered, the Holmake process then attempted to again include that path
in the newly entered directory. This is fine if the path is
absolute (the loop detection will stop the recursion), but if it's
relative, the path won't be present in that directory and a
file-system IO error occurs.

Closes #413

Make j1 Poly build pass on environment variable to sub-shells

Also rename reloc_build to relocbuild internally (making SML variable
name match command-line option name).

Add --relocbuild option to Holmake (without acting on it)

Update monitor code to use LargeInt explicitly

This is necessary to cope with Poly/ML's forthcoming shift to the
default integer type being fixed-width. The change is already visible
in the git repository's master branch.

Allow users to specify time-limit on command-line

Thus,

Holmake -t10

will kill any sub-process (e.g., a script file running) that doesn't do
anything (= produce any output) for 10 seconds.

Provide CLINE_OPTIONS variable in Holmakefiles

This variable is read for a set of default command-line switches which
are treated as if they are being passed on the command-line. Things
actually passed on the command-line are applied after what is in the
Holmakefile, possibly overriding the file's switches. Of course, the
command-line can also change the defaults with both of the
--no_hmakefile and --holmakefile=file switches.

Motivated by the desire to have Holmakefiles be able to contain good -j
options for specific directories.

This pretty well supercedes the OPTIONS variable, so the documentation
in DESCRIPTION has been adjusted accordingly.

Closes #86

Make Poly/ML Holmake use GetOpt

This allows it to also share "core" command-line options with the Moscow
ML implementation.

Progress with Github issue #77