Add 3 variants of _nm_utils_hexstr2bin*():

  - _nm_utils_hexstr2bin_full(), which takes a preallocated
    buffer and fills it.
  - _nm_utils_hexstr2bin_alloc() which returns a malloc'ed
    buffer
  - _nm_utils_hexstr2bin_buf(), which fills a preallocated
    buffer of a specific size.