mosh: add notes to clarify runtime issues

Added notes to the mosh package help to clarify some runtime
issues related with it to ensure mosh will work properly after adding it.
This includes adding a proper LOCALE and an extra flag when using
mosh with dropbear.

Signed-off-by: Gonçalo Salazar <glbsalazar@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>