Refactor query cancellation code into src/fe_utils/

Originally, this code was duplicated in src/bin/psql/ and
src/bin/scripts/, but it can be useful for other frontend applications,
like pgbench.  This refactoring offers the possibility to setup a custom
callback which would get called in the signal handler for SIGINT or when
the interruption console events happen on Windows.

Author: Fabien Coelho, with contributions from Michael Paquier
Reviewed-by: Álvaro Herrera, Ibrar Ahmed
Discussion: https://postgr.es/m/alpine.DEB.2.21.1910311939430.27369@lancre

src/include/fe_utils/cancel.h

@@ -0,0 +1,30 @@
+/*-------------------------------------------------------------------------
+ *
+ * Query cancellation support for frontend code
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/fe_utils/cancel.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef CANCEL_H
+#define CANCEL_H
+
+#include "libpq-fe.h"
+
+extern bool CancelRequested;
+
+extern void SetCancelConn(PGconn *conn);
+extern void ResetCancelConn(void);
+
+/*
+ * A callback can be optionally set up to be called at cancellation
+ * time.
+ */
+extern void setup_cancel_handler(void (*cancel_callback) (void));
+
+#endif							/* CANCEL_H */