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
wait on sndfd in nn_sock_send instead of rcvfd
Merge pull request #302 from TTimo/ipc_stress_windows_3

wait on sndfd in nn_sock_send instead of rcvfd
Fix many typos.
Merge pull request #304 from waywardmonkeys/typo-fixes

Fix many typos.