libusb_get_device_string() exposes only the device-descriptor strings
(manufacturer, product, serial number).  Applications also need the
configuration (iConfiguration) and interface (iInterface) strings, e.g.
to present a human-readable interface name during device selection
(#1855).

Add two public APIs that mirror libusb_get_device_string() but for
these descriptor-scoped strings, retrievable without opening the
device:

    int libusb_get_config_string(libusb_device *dev,
        uint8_t config_value, char *data, int length);
    int libusb_get_interface_string(libusb_device *dev,
        uint8_t config_value, uint8_t interface_number,
        uint8_t alt_setting, char *data, int length);

config_value is the bConfigurationValue, or 0 to select the currently
active configuration (0 is never a valid bConfigurationValue).  As in
libusb_get_device_string(), data may be NULL (with a non-zero length)
to query the required buffer size, and the returned string length
includes the NUL terminator.  LIBUSB_API_VERSION is bumped to
0x0100010D; the functions first appear in 1.0.31.

USB string descriptors are device-global and addressed by index, so
the fetch is identical to the device strings; only resolving
iConfiguration / iInterface from the cached configuration descriptor
is new.  That resolution is shared in usbi_get_config_string_index() /
usbi_get_interface_string_index(), and every backend resolves the
index first, so a nonexistent configuration/interface/alternate
setting or a missing string (index 0) fails identically everywhere
(LIBUSB_ERROR_NOT_FOUND) before any platform limit applies
(LIBUSB_ERROR_NOT_SUPPORTED).  The public wrappers fetch into an
internal scratch buffer and reuse usbi_utf8_copy() for
UTF-8-boundary-safe truncation, matching libusb_get_device_string()
minus its persistent cache, which cannot index the unbounded
config/interface/alt-setting space; results are therefore not cached,
as documented.  Because the core always passes that valid scratch
buffer, backends assert data/length rather than validate them.

Backends:
- windows (winusb): the parent-hub IOCTL fetch is extracted from
  winusb_get_device_string() into winusb_fetch_string_descriptor()
  (defined after the getters, so the original function and its
  explanatory comment stay in place) and shared by all three string
  getters.
- darwin: a GET_DESCRIPTOR control request; IOKit does not cache
  interface/configuration strings as IORegistry properties.  The
  device lock is held across the whole open/request/close sequence so
  that the open_count check stays stable, a concurrent libusb_open() /
  libusb_close() cannot interleave with the temporary open (plain
  USBDeviceOpen(), never Seize), and the device interface cannot be
  swapped by a re-enumeration.  The primary language ID is cached in
  the cached device, as the winusb backend does.
- linux: reads the named sysfs attributes (configuration,
  <dev>:<config>.<interface>/interface), sharing the read/strip helper
  with op_get_device_string() (which as a side effect now returns the
  length including the NUL terminator; its only caller checks the
  result solely for error).  Only the active configuration and the
  currently selected alternate setting are available without opening
  the device; other requests return LIBUSB_ERROR_NOT_SUPPORTED, as
  spelled out in the per-platform documentation notes.
- backends without an implementation return
  LIBUSB_ERROR_NOT_SUPPORTED.

The listdevs example prints the configuration and per-interface
strings in --verbose mode.

Assisted-by: claude-code:claude-opus-4-8