Ensure we discard unread/unsent data when abandoning a connection attempt.

There are assorted situations wherein PQconnectPoll() will abandon a
connection attempt and try again with different parameters (eg, SSL versus
not SSL).  However, the code forgot to discard any pending data in libpq's
I/O buffers when doing this.  In at least one case (server returns E
message during SSL negotiation), there is unread input data which bollixes
the next connection attempt.  I have not checked to see whether this is
possible in the other cases where we close the socket and retry, but it
seems like a matter of good defensive programming to add explicit
buffer-flushing code to all of them.

This is one of several issues exposed by Daniel Farina's report of
misbehavior after a server-side fork failure.

This has been wrong since forever, so back-patch to all supported branches.

src/interfaces/libpq/fe-connect.c

@@ -2068,6 +2068,9 @@ keep_going:						/* We will come back to here until there is
 						closesocket(conn->sock);
 						conn->sock = -1;
 						conn->status = CONNECTION_NEEDED;
+						/* Discard any unread/unsent data */
+						conn->inStart = conn->inCursor = conn->inEnd = 0;
+						conn->outCount = 0;
 						goto keep_going;
 					}
 					else
@@ -2105,6 +2108,9 @@ keep_going:						/* We will come back to here until there is
 						closesocket(conn->sock);
 						conn->sock = -1;
 						conn->status = CONNECTION_NEEDED;
+						/* Discard any unread/unsent data */
+						conn->inStart = conn->inCursor = conn->inEnd = 0;
+						conn->outCount = 0;
 						goto keep_going;
 					}
 				}
@@ -2218,6 +2224,9 @@ keep_going:						/* We will come back to here until there is
 						closesocket(conn->sock);
 						conn->sock = -1;
 						conn->status = CONNECTION_NEEDED;
+						/* Discard any unread/unsent data */
+						conn->inStart = conn->inCursor = conn->inEnd = 0;
+						conn->outCount = 0;
 						goto keep_going;
 					}
 
@@ -2285,6 +2294,9 @@ keep_going:						/* We will come back to here until there is
 						closesocket(conn->sock);
 						conn->sock = -1;
 						conn->status = CONNECTION_NEEDED;
+						/* Discard any unread/unsent data */
+						conn->inStart = conn->inCursor = conn->inEnd = 0;
+						conn->outCount = 0;
 						goto keep_going;
 					}
 
@@ -2304,6 +2316,9 @@ keep_going:						/* We will come back to here until there is
 						closesocket(conn->sock);
 						conn->sock = -1;
 						conn->status = CONNECTION_NEEDED;
+						/* Discard any unread/unsent data */
+						conn->inStart = conn->inCursor = conn->inEnd = 0;
+						conn->outCount = 0;
 						goto keep_going;
 					}
 #endif
@@ -2467,6 +2482,9 @@ keep_going:						/* We will come back to here until there is
 							closesocket(conn->sock);
 							conn->sock = -1;
 							conn->status = CONNECTION_NEEDED;
+							/* Discard any unread/unsent data */
+							conn->inStart = conn->inCursor = conn->inEnd = 0;
+							conn->outCount = 0;
 							goto keep_going;
 						}
 					}