Avoid undefined behavior when serializing malformed PLIST_DATE values
containing NaN, infinity, or values outside the Time64_T range. Add a
shared helper for checked date conversion and use it across writer paths.

Improve robustness and memory safety in node_to_string() and dtostr():

- add missing NULL and allocation checks
- fix snprintf() error handling and signed/unsigned conversions
- replace sprintf() with snprintf()
- fix base64 buffer sizing
- switch string offset tracking to size_t
- improve malformed data handling for strings and data blobs

Credit to @Bri1987

Convert all array/dict modification functions from void to plist_err_t return type:
  - plist_array_set_item: replace at index n
  - plist_array_append_item: append to end
  - plist_array_insert_item: insert at position n
  - plist_array_remove_item: remove item at index n
  - plist_array_item_remove: remove item from its array parent
  - plist_dict_set_item: replace or insert key/value
  - plist_dict_remove_item: remove key/value pair
  - plist_dict_merge: merge source dict into target

Returns:
  - PLIST_ERR_SUCCESS on success
  - PLIST_ERR_INVALID_ARG for invalid arguments (NULL, wrong type, out of range, etc.)
  - PLIST_ERR_NO_MEM on memory allocation failure
  - PLIST_ERR_UNKNOWN on unexpected internal errors

Header documentation updated with full error code semantics for each function.

- 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