Log Message:
Update mathomatic to 11.6c.

Changes:
- Some more touch-ups to solve routine.
- Changed uf_repeat() to skip expanding if the expression would be too big.
  This allows Mathomatic to work with huge exponents.
- Turned on "preserve_roots" option by default.
- Improved sum and product commands.  Allow counting down.
- Improved solve routine and removed display of useless messages.
- Added a fix to the limit command.
- Moved "source.txt" into "README.txt".
- Added code to simplify (inf+a) to (inf) generally.
- Fixed minor bug in calculate command.
- Minor fixes to elim_k().
- Improved limit command.
- Made all recursive functions in the source code static.
  Most functions never used globally defined as static.
- Improvement in command argument parsing.
- Added extrema command.
- Improved output of roots command.
- Fixed minor bug caused by "preserve_roots" option.
- Made file "am.htm" valid HTML.
- Simplification improved.