shared: Move buxton's private header to the shared lib
core: Move struct only locally used out of header

Since the client_list_item struct has no use outside of core/main.c
move the definition inside that file.
shared: Remove definitions for internal functions

Internal header definitions for functions only called locally to the
file were unneeded so remove them.
libbuxton: Move buxton_direct_open to external lib header
shared: Move private function defintion to shared util header
shared: Move util function defintions to util header
shared: Move backend defintions to their own header
shared: Split serialization functions into their own files
shared: Removed bt-daemon-private header

Since function defintions have been moved to more specific files,
remove no longer used generic header file.