We already have nm_utils_bin2hexstr() and _nm_utils_bin2hexstr_full().
This is confusing.

  - nm_utils_bin2hexstr() is public API of libnm. Also, it has
    a last argument @final_len to truncate the string at that
    length.
    It uses no delimiter and lower-case characters.

  - _nm_utils_bin2hexstr_full() does not do any truncation, but
    it has options to specify a delimiter, the character case,
    and to update a given buffer in-place. Also, like
    nm_utils_bin2hexstr() and _nm_utils_bin2hexstr() it can
    allocate a new buffer on demand.

  - _nm_utils_bin2hexstr() would use ':' as delimiter and make
    the case configurable. Also, it would always allocate the returned
    buffer.

It's too much and confusing. Drop _nm_utils_bin2hexstr() which is internal
API and just a wrapper around _nm_utils_bin2hexstr_full().