SDRplay support, rewritten, supporting RSP1, RSP1A, and RSP2

Requires ENABLE_NONFREE=yes. Requires SDRplay's SDK v2.11 to be installed by the user. In the future, if there is a way to redistribute mirsdrapi-rsp.h from the SDK, that problem could be addressed using a shim library that looks for /usr/local/lib/libmirsdrapi-rsp.so at runtime.