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

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

Write c/line option handling for mosml Holmake

This is as yet unused.