Support async pgsql connections and non-blocking queries

- New functions (each accepts a pgsql $connection resource):

  . pg_connect_poll
  . pg_socket
  . pg_consume_input
  . pg_flush

- Modified functions

  The following functions now additionally return zero if the
  underlying socket is set to non-blocking mode and the send
  operation does not complete immediately. Previously these
  functions returned only boolean TRUE/FALSE and blocked
  execution while polling until all data was sent:

  . pg_send_execute
  . pg_send_prepare
  . pg_send_query
  . pg_send_query_params

- New constants

  Used with pg_connect() to initiate an asynchronous connection
  attempt:

  . PGSQL_CONNECT_ASYNC

  Used with pg_connection_status() to determine the current state
  of an async connection attempt:

  . PGSQL_CONNECTION_STARTED
  . PGSQL_CONNECTION_MADE
  . PGSQL_CONNECTION_AWAITING_RESPONSE
  . PGSQL_CONNECTION_AUTH_OK
  . PGSQL_CONNECTION_SSL_STARTUP
  . PGSQL_CONNECTION_SETENV

  Used with pg_connect_poll() to determine the result of an
  async connection attempt:

  . PGSQL_POLLING_FAILED
  . PGSQL_POLLING_READING
  . PGSQL_POLLING_WRITING
  . PGSQL_POLLING_OK
  . PGSQL_POLLING_ACTIVE

- Polling via returned pg_socket() stream

  pg_socket() returns a read-only socket stream that may be
  casted to a file descriptor for select (and similar) polling
  operations. Blocking behavior of the pgsql connection socket
  can be controlled by calling stream_set_blocking() on the
  stream returned by pg_socket().