[sdb] Add a new 'socket-fd' transport, this is the same as the 'dt_socket' transport, but the address is the number of an an already connected socket fd.
[monodoc] Clarified message when a backing file isn't found

If the backing file doesn't exist, it will be ignored.
The previous message made it sound like this is a serious problem, which confused people as it is the last message that is
printed when building Mono (because of a reference to 'DoesNotExist' in docs/toc.xml) while in reality everything is fine.