calc: C-style arbitrary precision system Files

The following are the changes in this release:

    Added the makefile variable ${COMMON_ADD} that will add flags
    to all compile and link commands. The ${COMMON_ADD} flags are
    appended to both ${COMMON_CFLAGS} and ${COMMON_LDFLAGS}.  This
    facility is available to nearly all platforms except those with
    very old make commands that do not understand the += operator.

    Example on macOS (Darwin), one may invoke clang's -fsanitize
    facility by:

	make clobber all \
	  COMMON_ADD='-fsanitize=undefined -fsanitize=address'

    Another example.  To force C warnings to be treated as errors:

	make COMMON_ADD='-Werror'

    Created a GitHub repository for calc:

	https://github.com/lcn2/calc

    NOTE: The calc GitHub repository represents the an active
	  development stream.  While an effort will be made to keep
	  the master branch of the calc GitHub repository in working
	  order, that tree may be unstable.  Those wishing for more
	  reliable releases use releases found at calc mirror sites:

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

    IMPORTANT NOTE:

	On 2017 June 05, the calc GitHub history was re-written.
	Anyone who was tracking the calc "pre-release" on GitHub prior
	to version 2.12.6.0 should do a:

	    git reset --hard origin/master
	    git cleanup -f

	Or you may just want to start over:

	    rm -rf calc
	    git clone https://github.com/lcn2/calc.git

	Sorry about that.  The previous GitHub repository was an useful
	experiment.  Based on what we learned, we decided to rebuild it.

    Renamed README to README.FIRST.  Added README.md for the
    GitHub repository.

    Fixed reading from standard input (stdin) when -p is given on
    the command line.  This now prints hello:

	echo hello | calc -p 'stdin = files(0); print fgetline(stdin);'

    Added more debugging related to stdin when bit 4 of calc_debug
    is set (e.g., running calc with -D16).

    Updated the calc(1) man page and 'help file' to explain about
    reading from standard input (stdin).

    Added some clarifying remarks for 'help ptest' explaining that
    the ptest builtin can return 1 is some cases where the test
    value is a pseudoprime.

    Removed duplicate copyright comments from the help/builtin that
    is built.

    Fixed a number of typos in the CHANGES file.