ScriptRunner: Show error when write() to fd fails
Add new method umockdev_testbed_load_socket_script

This will replay recorded read()s/recv()s and write()s/send()s to a Unix
socket.

As Unix sockets behave quite differently from tty devices (one listen/many
accept server-side sockets, send/recv, etc.) we can't integrate them into the
existing dev_fd/dev_script_runner data structures but have to add a new
object/thread SocketServer that listens on the socket and creates a
ScriptRunner object for each accepted connection.

Note that we do not implement recvfrom()/sendto() for now, just as in commit
ef7885.

https://github.com/martinpitt/umockdev/issues/24