Log Message:
Update mathomatic to 12.5.5.

Changes:
- Added "doc/manpage.html", which is the man page for Mathomatic
  generated with "groff -Thtml -man".
- Made sure all HTML files have the extension ".html".
- Made sure we can allocate at least 1 equation space on startup.
- Made entering things like "#1 a" succeed at entering a single variable
  expression.
- Fix-ups to the symbolic math library.
- Increased power of limit command.
- A limit expression with infinity in it is assumed to be infinity.