Small rst.el stuff.

Small rst.el stuff.

* lisp/textmodes/rst.el (rst-compile-toolsets): Make it a defcustom.
Add `pdf' and `s5' entries.  Use `prog.py' if found, otherwise
default to `prog' without a .py extension.
(rst-compile-pdf-preview, rst-compile-slides-preview):
Use program names from rst-compile-toolsets, rather than hard-coding.
(rst-portable-mark-active-p): Fix presumed typo.