Restructure incomplete or broken "autoconf" implementations

Some plugins with non-zero autoconf exitcodes (it must be zero instead)
deserved a bit of code cleanup for improved readability.

plugins/mythtv/dvb-signal

@@ -18,19 +18,15 @@ Tuner=0
 
 case $1 in
     autoconf|detect)
-       REQ=`which femon`
-       if [ "$REQ" = "" ]; then
+       if ! which femon >/dev/null; then
           echo "no (femon not installed)"
-          exit 1
-       fi
-
-       if [ -e  /dev/dvb/adapter$Tuner/ ]; then
+       elif [ -e "/dev/dvb/adapter$Tuner/" ]; then
           echo yes
-          exit 0
        else
           echo "no (dvb tuner $Tuner not found)"
-          exit 1
-       fi;;
+       fi
+       exit 0
+       ;;
 
     config)
       echo "graph_title  DVB signal strength"