calc: C-style arbitrary precision system Files

The following are the changes in this release:

    Fixed typo (missing quotes) in the env rule.

    Fixed intendation problem in CHANGES.

    Combined 2.12.9.1 changes into the 2.12.8.2 to 2.12.9.0
    range, and thus renamed the range to 2.12.8.2 to 2.12.9.1.

    Fixed issues related to building Makefile.simple.

    Fixed how the Makefile variable MANPATH is set for macOS.

    Added a bunch of information to the near bottom of HOWTO.INSTALL
    on calc Makefiles.  This information discusses the various
    Makefiles found in the calc source.

    Added comments in various calc Makefiles about their origin.
    In particular, for Makefiles that are constructed such as
    Makefile.simple, custom/Makefile and custom/Makefile.simple
    there are comments about how they were made.

    For all calc Makefiles, including those in sub-directories,
    near the top there is now a line of the form:

	# SRC: ... some message about the origin ...

    Fixed how the calc(1) man page is installed under macOS.

    Fixed how calc man page in ${CATDIR} is formed.

    Fixed how Makefile.simple is formed.

    Fixed the #! calc script argument processing.  The initial #!
    line must end in a -f.  For example, if calc is in /usr/local/bin/calc,
    then the following would be the first line of a calc script:

	#!/usr/local/bin/calc -f
	...

    It is common that -q be usde with a calc script, so assuming the
    same /usr/local/bin/calc path:

	#!/usr/local/bin/calc -q -f
	...

    Use of -s in the #! first line of a calc script is not needed
    since -f implies -f.

    The argv() will now return values more typical of C's main().
    Before it returned one less than the number of arguments.  Now,
    for example, when calc is given 2 args, argv() will return 3.

    The value of argv(0) will be the path to calc, or in the
    case of a #! calc cscript, it will return the name of the script.

    Updated the calc man page and help/argv to reflect the
    above changes.

    Improved the formatting of the calc man page.

    Fixed the formation of the win32 sub-directory via the win32_hsrc
    Makefile rule.

    Due to incompatible changes to the argv() function, and #! calc
    scripts, we are setting the version to the next minor number:

	2.13.0