- Use PLIST_OPT_COERCE option to coerce PLIST_BOOLEAN, PLIST_DATE, PLIST_UID, and PLIST_NULL to OpenStep-compatible types (1 or 0, ISO 8601 strings, integers, and 'NULL' string)
- Add plist_to_openstep_with_options() function to allow passing coercion option (and others)
- Update plist_write_to_string() and plist_write_to_stream() accordingly

- Add PLIST_OPT_COERCE option to coerce PLIST_DATE, PLIST_DATA, and PLIST_UID to JSON-compatible types (ISO 8601 strings, Base64 strings, and integers)
- Add plist_to_json_with_options() function to allow passing coercion options (and others)
- Update plist_write_to_string() and plist_write_to_stream() to support coercion option
- Add --coerce flag to plistutil for JSON output
- Create plist2json symlink that automatically enables coercion when invoked

Introduce private iterator structs for plist_array_iter and
plist_dict_iter, and fix *_next_item() to properly advance
iterator state and handle malformed containers safely.

Thanks to @unbengable12 for reporting. Addresses #288, #289, #290, #291, and #292.

Thanks to @LkkkLxy for pointing out the issue.

These functions work with int64_t values representing a UNIX timestamp instead
of using the 'MAC epoch'. They should be used instead of plist_new_date,
plist_get_date_val, and plist_set_date_val, which are now marked deprecated
and might be removed in a future version of libplist.