fix multicastReceiver socket leak when unable to set up socket

bracketOnError the socket setup, so that the socket can be closed if setup
fails. The exception is still propigated to the caller as before, the only
difference is the file descriptor leak is fixed. Fixes issue #4

Also fixed issue #5, since I cannot test without it. Avoid trying to use
SO_REUSEPORT on Linux, since it fails with Linux 3.10 here. It's not clear
why the linux kernel headers define SO_REUSEPORT if the kernel does not
support it. This may change in a future version of Linux or possibly Glibc.