Fix query cancellation handling in psql

The refactoring done in a4fd3aa for query cancellation has messed up
with the logic in psql by mixing CancelRequested and cancel_pressed,
breaking for example \watch.  The former would be switched to true if a
cancellation request has been attempted and that it actually succeeded,
and the latter tracks if a cancellation attempt has been done.

This commit brings back the code of psql to a state consistent to what
it was before a4fd3aa, without giving up on the refactoring pieces
introduced.  It should be actually possible to merge more both flags as
their concepts are close enough, however note that psql's --single-step
mode relies on cancel_pressed to be always set, so this requires more
careful analysis left for later.

While on it, fix the declarations of CancelRequested (in cancel.c) and
cancel_pressed (in psql) to be volatile sig_atomic_t.  Previously,
both were declared as booleans, which should be fine on modern
platforms, but the C standard recommends the use of sig_atomic_t for
variables used in signal handlers.  Note that since its introduction in
a1792320, CancelRequested declaration was not volatile.

Reported-by: Jeff Janes
Author: Michael Paquier
Discussion: https://postgr.es/m/CAMkU=1zpoUDGKqWKuMWkj7t-bOCaJDx0r=5te_-d0B2HVLABXg@mail.gmail.com

src/fe_utils/cancel.c

@@ -16,7 +16,6 @@
 
 #include "postgres_fe.h"
 
-#include <signal.h>
 #include <unistd.h>
 
 #include "fe_utils/cancel.h"
@@ -37,8 +36,20 @@
 		(void) rc_; \
 	} while (0)
 
+/*
+ * Contains all the information needed to cancel a query issued from
+ * a database connection to the backend.
+ */
 static PGcancel *volatile cancelConn = NULL;
-bool		CancelRequested = false;
+
+/*
+ * CancelRequested tracks if a cancellation request has completed after
+ * a signal interruption.  Note that if cancelConn is not set, in short
+ * if SetCancelConn() was never called or if ResetCancelConn() freed
+ * the cancellation object, then CancelRequested is switched to true after
+ * all cancellation attempts.
+ */
+volatile sig_atomic_t CancelRequested = false;
 
 #ifdef WIN32
 static CRITICAL_SECTION cancelConnLock;