calc: C-style arbitrary precision system Files

The following are the changes in this release:

    Fixed a mistake in "help intro" where some inserted text changed
    the value of "." and thus made the next result incorrect.

    Clarified in "help factor" that 1 is returned if no
    factor below the limit was found.

    Removed Makefile variable ${MAKEFILE_REV}.

    The missing cscript/square.calc file has been restored.

    Fixed compiler errors and warnings related to GCC.  In particular,
    gcc/9.3.1 and gcc/10.2.1 now compile calc without any compiler
    errors or warnings, even with -Werror -Wextra -pedantic.

    To print out information about the calc compilation
    environment, we added the following make rule:

	make calcinfo

    Improved how 'make debug' operates.

    Moved help/contrib to CONTRIB-CODE.  The help/contrib file
    is now build from a copy of CONTRIB-CODE.

    Created a new calc bug report Email address.  Created a new
    calc question Email address.  Created a new calc contribution
    Email address.  See the BUGS file for details.

    Added "help questions" to print the QUESTIONS help file.

    If the environment variable $CALCHELP is defined and is non-empty,
    then calc help files will be in the directory by the $CALCHISTFILE
    environment variable.

    If the environment variable $CALCCUSTOMHELP is defined and is
    non-empty, then custom calc help files will be in the directory
    by the $CALCCUSTOMHELP environment variable.

    Fixed a number of typos in text and in source code comments.

    The calc-tester mailing list has been retired.  See:

	* How to submit a calc bug report:

	    http://www.isthe.com/chongo/tech/comp/calc/calc-bugrept.html

	* How to contribute code to calc:

	    http://www.isthe.com/chongo/tech/comp/calc/calc-contrib.html

	* How to submit a question about calc:

	    http://www.isthe.com/chongo/tech/comp/calc/calc-question.html