Allow using non-standard usbmuxd socket address via environment variable

By using USBMUXD_SOCKET_ADDRESS environment variable, it is possible
to make libusbmuxd connect to the specified address. The value needs
to be in format ADDRESS:PORT (or UNIX:PATH on unix systems). If no port
number is specified or parsing fails, the standard socket address (or
unix domain socket file path) will be used silently.