dbLink: Clarify meaning of lset isConstant and isVolatile flags

Modify dbIsLinkConnected() to check lset->isVolatile first.

src/ioc/db/dbLink.c

@@ -265,8 +265,19 @@ int dbIsLinkConnected(const struct link *plink)
 {
     lset *plset = plink->lset;
 
-    if (!plset || !plset->isConnected)
+    if (!plset)
         return FALSE;
+    if (!plset->isVolatile)
+        return TRUE;
+
+    if (!plset->isConnected) {
+        struct dbCommon *precord = plink->precord;
+
+        errlogPrintf("dbLink: Link type for '%s.%s' is volatile but has no"
+            " lset::isConnected() method\n",
+            precord->name, link_field_name(plink));
+        return FALSE;
+    }
 
     return plset->isConnected(plink);
 }

src/ioc/db/dbLink.h

@@ -31,8 +31,8 @@ typedef long (*dbLinkUserCallback)(struct link *plink, void *priv);
 
 typedef struct lset {
     /* Characteristics of the link type */
-    const unsigned isConstant:1;
-    const unsigned isVolatile:1;
+    const unsigned isConstant:1;    /* 1 means value doesn't change */
+    const unsigned isVolatile:1;    /* 0 means link always connected */
 
     /* Activation */
     void (*openLink)(struct link *plink);