Replace "which" with "command -v".

Split2flac is incompatible with the [dash](http://gondor.apana.org.au/~herbert/dash/) (a POSIX compliant `/bin/sh` implementation) due to the use of the `&>` output redirection operator in conjunction with the `which` commands.  In order to fix this, the `&>` operator has been replaced by `>`, and (in order to avoid the outputs to stderr) `which` has been replaced by `command -v`.

Note that this could alternatively be solved by replacing the use of `&>/dev/null` with `>/dev/null 2>&1`, but I didn't like that as much.