string_helpers: Move string_is_valid() to the header

Move string_is_valid() to the header for wider use.

While at it, rename to string_is_terminated() to be
precise about its semantics.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230208133153.22528-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>