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

pidof.1

@@ -15,17 +15,17 @@
 .\" along with this program; if not, write to the Free Software
 .\" Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 .\"
-.TH PIDOF 1 "2020-06-04" "" "User Commands"
+.TH PIDOF 1 "2020-12-22" "" "User Commands"
 .SH NAME
 pidof -- find the process ID of a running program
 .SH SYNOPSIS
 .B pidof
 .RB [ \-s ]
 .RB [ \-c ]
+.RB [ \-q ]
+.RB [ \-w ]
 .RB [ \-x ]
 .RB [ \-o
-.IR omitpid[,omitpid...] ]
-.RB [ \-o
 .IR omitpid[,omitpid...]... ]
 .RB [ \-S
 .IR separator ]
@@ -42,12 +42,14 @@ Single shot - this instructs the program to only return one \fIpid\fP.
 Only return process ids that are running with the same root directory.
 This option is ignored for non-root users, as they will be unable to check
 the current root directory of processes they do not own.
-.IP \-x
-Scripts too - this causes the program to also return process id's of
-shells running the named scripts.
+.IP \-q
+Quiet mode, suppress any output and only sets the exit status accordingly.
 .IP \-w
 Show also processes that do not have visible command line (e.g. kernel
 worker threads).
+.IP \-x
+Scripts too - this causes the program to also return process id's of
+shells running the named scripts.
 .IP "-o \fIomitpid\fP"
 Tells \fIpidof\fP to omit processes with that process id. The special
 pid \fB%PPID\fP can be used to name the parent process of the \fIpidof\fP