rarpd: clarify synopsis

[Klemens Nanni]

rarpd(8) either "Listen[s] on all the Ethernets attached to the system"
or requires an explicit list, not both:

        $ rarpd -a em0
        usage: rarpd [-adflt] if0 [... ifN]
        $ ./obj/rarpd -a em0
        usage: rarpd [-dflt] -a | if ...

Or would this be better?
        rarpd [-dflt] if ...
        rarpd -a [-dflt]

Feedback? OK?

Index: rarpd.8
===================================================================
RCS file: /cvs/src/usr.sbin/rarpd/rarpd.8,v
retrieving revision 1.21
diff -u -p -r1.21 rarpd.8
--- rarpd.8     28 Oct 2015 10:02:59 -0000      1.21
+++ rarpd.8     29 Sep 2022 21:04:29 -0000
@@ -29,8 +29,8 @@
 .Nd reverse ARP daemon
 .Sh SYNOPSIS
 .Nm rarpd
-.Op Fl adflt
-.Ar if0 Op Ar ... ifN
+.Op Fl dflt
+.Fl a | Ar if ...
 .Sh DESCRIPTION
 .Nm
 services Reverse ARP requests on the Ethernet connected to
Index: rarpd.c
===================================================================
RCS file: /cvs/src/usr.sbin/rarpd/rarpd.c,v
retrieving revision 1.79
diff -u -p -r1.79 rarpd.c
--- rarpd.c     15 Nov 2021 15:14:24 -0000      1.79
+++ rarpd.c     29 Sep 2022 21:05:35 -0000
@@ -217,7 +217,7 @@ init_all(void)
 __dead void
 usage(void)
 {
-       (void) fprintf(stderr, "usage: rarpd [-adflt] if0 [... ifN]\n");
+       fprintf(stderr, "usage: rarpd [-dflt] -a | if ...\n");
        exit(1);
 }