nn_pollfd typo

nn_polld -> nn_pollfd
[udp rfc] Don't refer to a connected state.

Since UDP is connectionless, refer to an application sending data
rather than an application being connected.
[udp rfc] Fix some typos.
Merge pull request #301 from waywardmonkeys/udp-rfc-wording

Udp rfc wording
Merge pull request #293 from Kerrigan29a/patch-1

nn_pollfd typo
Merge branch 'master' of https://github.com/nanomsg/nanomsg.git