Original commit was just to hold my place.

This now completes a UDS implementation

- Two new defines
CPPA_UDS_ENABLED              UDS network sockets available
CPPA_UDS_ABSTRACT_NAMESPACE   UDS has abstract namespace

- One new configure option

--no-uds                      overrides having UDS in the library
--no-uds_abstract             overrides allowing use of the abstract namespace

- Two new source files

 src/uds_acceptor.cpp
 src/uds_io_stream.cpp

Still to go:
- Need to add a unit test
- Need to add an example app