- for nm_utils_enum_to_str(), whenever encounter a numeric value
  that has no expression as enum/flag, encode the value numerically.
  For enums, encode it as decimal. For flags, encode it as hexadecimal
  (with 0x prefix).
  Also check that an existing value_nick cannot be wrongly interpreted
  as a integer, and if they would, encode them instead as integers only.

- Likewise, in nm_utils_enum_from_str() accept numerical values
  and for nm_utils_enum_get_values() return enum nicks that look
  like numeric values in their numeric form only.

- In nm_utils_enum_from_str(), don't use g_strsplit(), but clone the
  string only once and manipulate it inplace.

- Accept '\n' and '\r' as additional delimiters for flags.

- For consistency, also return an err_token for enum types. If the caller
  doesn't care about that, he should simply not pass the out-argument.