stun: Rename a symbol to avoid a naming clash with OpenSSL

The symbol is not exported, but nevertheless leaks in the static version
of libnice, due to limitations on symbol visibilities with static
libraries. OpenSSL has a symbol named RAND_bytes(), which ours clashes
with if the two are linked together statically.

Avoid this by prefixing ours with ‘nice_’.
agent: Close pseudo-TCP socket earlier in component_close()

This tries to mitigate the race condition between finishing the TCP FIN
handshake and closing the underlying sockets, but it’s impossible to
mitigate properly without API changes. See the comment.
agent: Expand simple example in documentation to mention GMainContext

wrt the ML thread:
http://lists.freedesktop.org/archives/nice/2014-October/000981.html
nicesink: unblock pending write on component change

When a nicesink switches to a different component, we should unblock any
pending write waiting in gst_nice_sink_render() for writable_cond being
signalled because if the new transport already is writable, we'll never
get notified about the condition and thus nicesink will wait forever.

That way, either the pending data are immediately sent or the nicesink
blocks again until the transport becomes writable.
interfaces: prune duplicate IPs
TEMP: Revert "agent: Avoid leaking UPnP mappings between streams"

This reverts commit 375cde8a652f8e367cf875bcb718a6d1abcfb254.

We don't have gupnp-igd 0.2.4 in the repository for this change to compile.