connection: Read more data per call to wl_connection_read()

Split off from !213 (closed).

recvmsg() doesn't necessarily read all available data from the socket, so call it in a loop until we've read all available data or filled the read buffer. This helps to avoid buffer bloat.

This is actually enough to fix weston#589 on its own.

I also included a small (non-functional) simplification to wl_connection_write().