Expose PQsocketPoll via libpq

Expose PQsocketPoll via libpq

This is useful when connecting to a database asynchronously via
PQconnectStart(), since it handles deciding between poll() and
select(), and some of the required boilerplate.

Tristan Partin, reviewed by Gurjeet Singh, Heikki Linnakangas, Jelte
Fennema-Nio, and me.

Discussion: http://postgr.es/m/D08WWCPVHKHN.3QELIKZJ2D9RZ@neon.tech