mailcap: extract the FILE parsing routine and export it so that we can write tests for this module.

update telescope to make use of mailcap Change telescope to make use of mailcap entries. Commands are no longer prompted for how to handle an unknown mime-type. Commands are now executed via execvp(), and the mailcap handling assumes this as part of its interface.

mailcap: add parser for external command handling This introduces a (limited) subset of mailcap handling, as defined in RFC 1524. This makes it possible for telescope to run commands on different mime types as defined in a mailcap file, but defaults to a default mime type of using xdg-open if no match is found, or no mailcap file defined.