Readline support for native scripting — Please review

These changes allow fontforge to be configured with readline support.  It really only has any impact if fontforge is configured --without-x (i.e. as a standalone script interpreter), but it makes the native scripting interface fully interactive, with command-line editing, history recall, and per-line execution with return value printed.  I've found it extremely useful for native script development, but given its experimental nature I would prefer that it be code reviewed and accepted into the fontforge master stream by someone other than myself.