Add config option to redirect external helpers error output to a file

src/sampleconf/recoll.conf

@@ -660,6 +660,17 @@ logfilename = stderr
 # <brief>Override logfilename for the indexer.</brief><descr></descr></var>
 #idxlogfilename = stderr
 
+# <var name="helperlogfilename" type="fn">
+#
+# <brief>Destination file for external helpers standard error
+# output.</brief>
+#
+# <descr>The external program error output is left alone by default,
+# e.g. going to the terminal when the recoll[index] program is executed
+# from the command line. Use /dev/null or a file inside a non-existent
+# directory to completely suppress the output.</brief></var>
+#helperlogfilename=
+
 # <var name="daemloglevel" type="int">
 #
 # <brief>Override loglevel for the indexer in real time