calc 2.12.6.8 release

The following are the changes in this release:

For historical purposes, in lucas.cal, gen_v1(1, n) always returns 4.

Fixed some compiler warnings, thanks to a report by Mike
<michael dot d dot ince at gmail dot com>.

Added work around for a gcc warning bug, thanks to a report by Mike
<michael dot d dot ince at gmail dot com>.

Fixed errors in various help files such as:

mat randbit seed srandom types

Removed the MAXSTRING symbol because it was no longer used by calc.

Increased HIST_SIZE (depth of the history stack) from 10k to 32k.

Increased TTYSIZE (reallocation size for terminal buffers) from 100 to 8191.

Increased MAXDEPTH (maximum depth of input stack) from 10 to 255.

Increased interactive input buffer size from 1024 to 256k.  This has the
effect of increasing the maximum length of an input line from a tty.
This helps with an interactive bug that was reported by Ruslan Kabatsayev
(b7 dot 10110111 at gmail dot com).

The calc man page indicates that -d also disables the printing of the
leading tilde.

Added information to "help command" about how to silence messages
while reading calc resource files.

Fixed an error message buffer overflow thanks to a report by
Frank Peters <nlp at northernlightsphoto dot biz>.

Replaced all use of the C funcion sprintf() with snprintf().
Replaced all use of the C funcion vsprintf() with vsnprintf().
Replaced all DONT_HAVE_VSPRINTF with DONT_HAVE_VSNPRINTF.
Replaced all Makefile var ${HAVE_VSPRINTF} with ${HAVE_VSNPRINTF}.