Exit from base backups when shutdown is requested

When the exit waits until the whole backup completes, it may take
a very long time.

In passing, add back an error check in the main loop so we detect
clients that disconnect much earlier if the backup is large.

src/include/replication/walsender.h

@@ -53,6 +53,8 @@ extern WalSndCtlData *WalSndCtl;
 
 /* global state */
 extern bool am_walsender;
+extern volatile sig_atomic_t walsender_shutdown_requested;
+extern volatile sig_atomic_t walsender_ready_to_stop;
 
 /* user-settable parameters */
 extern int	WalSndDelay;