FS-353: Fix fs_cli to work with all libedit versions

The old bundled version didn't properly support escape codes but we
worked around this. Newer versions do support escape codes and the
workaround no longer works.

Newer versions also have their own function for redisplaying the
prompt but this was unimplemented in the narrow character fallback
code until yesterday. Thankfully calling the wide character version
does the same thing.

Fortunately we have autoconf to figure this mess out! Unfortunately
coloured prompts are still slightly broken but this is being looked
into.