interpreter: Fix the bug where messages would sometimes be interpreted as commands (i.e. "aversion" was intepreted as "/version").
Initialize locale in init.scm to allow sending utf8 messages.
Make ft_roster_lookup behave correctly even when supplied with an empty string.

This allows the user to do (ft-set-current-buddy! "") in .scm scripts.
Previously, this would lead to a weird behaviour or even segfault.