Fix problem generating file names in send-to-device and possibly ...

...save-to-disk. The original code assumed that the last component always had an extension, which isn't true when evaluating templates to generate a file name. If the resulting string was too long and if last segment of the string contained a period then the remainder of the string after the period was assumed to be an extension. If that remainder was longer than permitted, shorten_components_to generated a path consisting of a single letter.

Example: if the template generated this string:
"Some of the Best From Tor.com, 2013 Edition_ A Tor.Com Original - A. B. C. Personn & Aa Angstrom & Anne McCaffrey & Banks, Iain M & Bruce Sterling & Cajkovskij, Petr Ilic & Chacko, David & Charlie Huston & Collins, Wilkie & D'Ansey, Leigh & Dahl, Roland & Edward Elmer _Doc_ Smith & Eric Flint & George Charon & Jules Verne (1422)"
then the "extension" was everything after the A. B. C., with a length that can easily exceed the max path len.