pidof: allow to suppress output

Often pidof is used in shell scripts in this form:

    if pidof daemon >/dev/null; then
        ...
    fi

The redirection to /dev/null is needed because otherwise the script
would output the found PIDs.
Let's add a -q option which, similary to grep, just sets the exit code.
Also exit on first match, as there is no reason to proceed further when
at least a process is matched.

Tested with:

    $ ./pidof bash
    17701 14019 5276 2967
    $ echo $?
    0
    $ ./pidof bashx
    $ echo $?
    1
    $ ./pidof -q bash
    $ echo $?
    0
    $ ./pidof -q bashx
    $ echo $?
    1
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