settings: shell: improve reading and writing string values

Make reading and writing string values more flexible:
1. Eliminate the intermediate buffer when saving a string
   setting. This needlessly limited the maximum string
   length that could be saved using the shell command.
2. Do not add nor assume that a string saved in the settings
   includes the null-terminator. The settings subsystem uses
   metadata for encoding the value length, so there it is
   redundant to also store the null-terminator in flash.

By the way, also make sure the command handlers only return
-EINVAL and -ENOEXEC error codes as documented in the
handler type description.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>