provide null-terminated `z_owned_sting_t` variant

[milyin]

The z_owned_stirng_t type contains string which is not null terminated. But It's very typical error to use z_string_data() with e.g. %s type specifier of for strcmp(). With AI-based code autocompletion it happens even more easily. We need to take some measures to prevent this kind of bugs:

Necessary actions:

• fix documentation:
  • say in doc for type z_owned_string_t itself that sting is NON-null terminated
  • say that z_owned_string_t is NON-null terminated in z_string_data()
  • rephrase z_string_len() help: phrase "the length of the string (without terminating 0 character)." is misleading
  • review docuentation

Proposed API update:

• upgrade z_owned_string_t type to allow it to store null-terminated strings:
  • add boolean flag "is null-terminated" (we can use one bit of size field for this to not extend the structure size)
  • add function const char* z_string_as_cstr(z_loaned_string_t* s). This function will either just return pointer to string data (if string is already null-terminated) or reallocate passed string s (notice that pointer in non-const) by adding zero to the end and setting the null-terminated flag.
  • add function z_string_is_cstr(const z_loaned_string_t*) which returns this flag

This update will provide a direct way to get null-terminated string which is the frequent need of C developers