installer/resources/i2prouter: various bugfixes

- Explicitly specify shell when RUN_AS_USER is set. This allows I2P to start
  under limited accounts that do not have a defined shell.
- When installing as a service, point out that I2P_CONFIG_DIR may need to be
  set. I2P_CONFIG_DIR defaults to $HOME/.i2p for the user that installed I2P
  and when running as a service this may not be desired.
  (e.g., RUN_AS_USER=i2p & I2P_CONFIG_DIR=/root/.i2p)

(The RUN_AS_USER shell problem was spotted at https://github.com/mattias-ohlsson/i2p/blob/master/i2p.spec)

installer/resources/i2prouter

@@ -702,9 +702,9 @@ checkUser() {
         #
         if test -f "/sbin/runuser"
         then
-            /sbin/runuser - $RUN_AS_USER -c "\"$REALPATH\" $ADDITIONAL_PARA"
+            /sbin/runuser -s /bin/sh - $RUN_AS_USER -c "\"$REALPATH\" $ADDITIONAL_PARA"
         else
-            su - $RUN_AS_USER -c "\"$REALPATH\" $ADDITIONAL_PARA"
+            su - $RUN_AS_USER -s /bin/sh -c "\"$REALPATH\" $ADDITIONAL_PARA"
         fi
         RUN_AS_USER_EXITCODE=$?
         # Now that we are the original user again, we may need to clean up the lock file.
@@ -1777,7 +1777,7 @@ showUsage() {
 }
 
 showsetusermesg()  {
-    echo "`gettext 'Please edit $0 and set the variable RUN_AS_USER'`."
+    echo "`gettext 'Please edit i2prouter and set the variable RUN_AS_USER'`."
 }
 
 checkifstartingasroot() {
@@ -1785,7 +1785,7 @@ checkifstartingasroot() {
         echo "`gettext 'Running I2P as the root user is *not* recommended.'`"
         showsetusermesg
         echo
-        echo "`gettext 'To run as root anyway, edit $0 and set ALLOW_ROOT=true.'`"
+        echo "`gettext 'To run as root anyway, edit i2prouter and set ALLOW_ROOT=true.'`"
         exit 1
     fi
 }
@@ -1853,11 +1853,16 @@ docommand() {
         'install')
             installdaemon
             if [ ! `grep ^RUN_AS_USER $0` ]; then
+                 echo
                  showsetusermesg
             fi
             echo
             echo "You may want to disable the browser from launching at startup at"
             echo "http://127.0.0.1:7657/configclients"
+            echo
+            echo "I2P_CONFIG_DIR is currently set to $I2P_CONFIG_DIR."
+            echo "Change the value in $0 if this is not"
+            echo "appropriate for your configuration."
             ;;
 
         'remove' | 'uninstall')