Fix test for existence of command

`which` will print, even if the command does not exist, so the test will
never fail. Additionally, `which` is non-standard, and has been
deprecated by debian. `command -v` is the POSIX-standardized way of
testing for existence of commands, and works the same even on non-POSIX
systems that implement the `command` command.

`command -v` will exit with 0 and print to stdout when the command
exists, and will exit with 1 and *not* print anything to stdout or
stderr when the command does not exist.

doc/scripts/init/rtorrentInitTmux.bash

@@ -80,7 +80,7 @@ SCRIPTNAME=/etc/init.d/$NAME
 RTXMLRPCBIN="$base/bin/rtxmlrpc"
 
 checkcnfg() {
-  if [ -z "$(which $DAEMON)" ] ; then
+  if ! command -v "$DAEMON" >/dev/null ; then
     echo "Cannot find $DAEMON binary in PATH: $PATH"
     exit 3
   fi