Retrieve declared descriptor size

Request transfer of report descriptor with the destination buffer size
equal to report descriptor size declared in HID descriptor, instead of
the maximum possible report descriptor size. Only fall back to the
maximum possible size, if report descriptor size cannot be retrieved.

This fixes ERROR_PIPE when retrieving report descriptors from some
devices (e.g. Waltop Vega tablet).
Mention bootstrap in installation instructions
Avoid unused rc variable warning

Ensure compiler assumes rc is used in uhd_iface_new to avoid unused
variable warning with NDEBUG builds.