Initial DSTAR AMBE voice decoding support

This commit goes together with https://github.com/szechyjs/mbelib/pull/10 to add DSTAR voice decoding support.

Some GPL licensed code borrowed from https://github.com/on1arf/gmsk was added, wholly in separate source files — the COPYING file was updated to indicate which source files contain this code. As the GPL is compatible with the BSD license, this is allowed, but the compiled final product is considered under the GPL.

This commit depends on the mbelib support added in https://github.com/szechyjs/mbelib/pull/10, as well as the dsd fixes in https://github.com/szechyjs/dsd/pull/5 and https://github.com/szechyjs/dsd/pull/6.