Generalize stream / socket related exceptions

This is in preparation for adding UDP sockets, as UDP sockets and TCP
sockets have no common superclass, as one is stream-oriented while the
other is packet-oriented.

Read and write exceptions are for any object now, as they are useful for
a lot more than just for streams, while the others (bind, listen, etc.)
are for any socket now (the type is id in this case, though, as there is
no common superclass).
Move socket includes and helpers to separate files

The new file socket.h includes all headers required for sockets on the
used platform, while the file socket_helpers.h defines the BSD API
functions to the platform specific functions if necessary.

This cleans up the classes dealing with sockets a lot and also reduces
code duplication.