Simplified paramLib access and show error messages for one poll cycle

Simplified getting and setting paramLib entries via a macro and created
a mechanism within poll() which makes sure that error messages are shown
for at least one poll cycle. Also moved MAXBUF_ to the SinqController
level.

src/sinqController.h

@@ -376,6 +376,11 @@ class epicsShareClass sinqController : public asynMotorController {
      */
     int outstandingForcedFastPolls() { return outstandingForcedFastPolls_; }
 
+    // Maximum error message buffer size. This is an empirical value which must
+    // be large enough to avoid overflows for all commands to the device /
+    // responses from it.
+    static const uint32_t MAXBUF_ = 200;
+
     // =========================================================================
 
   protected: