db: Allow single-char accesses as long strings

Clients try to use long string support to fetch DBF_STRING
fields use DBF_CTRL_CHAR with a 1-element array, but the IOC
was rejecting that.  This permits it, and also ensures that
the resulting strings are zero-terminated.

Fixes lp:907761

documentation/RELEASE_NOTES.html

@@ -13,6 +13,18 @@
 
 <!-- Insert new items immediately below here ... -->
 
+<h4>Launchpad Bugs Resolved</h4>
+
+<p>The following are links to bugs in the Launchpad bug tracker that have been
+fixed in this release:</p>
+
+<ul>
+<li>907761
+  <a href="https://launchpad.net/bugs/907761">
+    reading only 1st char of link in "long string" ($) syntax fails in read
+    error</a></li>
+</ul>
+
 <h4>Comments in iocsh scripts</h4>
 
 <p>The IOC shell was very particular about comments in previous versions of