- 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.