vnconfig: fix usage

[Klemens Nanni]

The manual has it right about -k and -K being mutually exclusive;
adapt the internal text.

Merge the printf calls while here.

OK?

Index: vnconfig.c
===================================================================
RCS file: /cvs/src/sbin/vnconfig/vnconfig.c,v
retrieving revision 1.8
diff -u -p -r1.8 vnconfig.c
--- vnconfig.c  16 Aug 2022 16:18:16 -0000      1.8
+++ vnconfig.c  19 Aug 2022 18:24:37 -0000
@@ -356,11 +356,9 @@ __dead void
 usage(void)
 {
        fprintf(stderr,
-           "usage: vnconfig [-kv] [-K rounds] [-S saltfile] "
-           "[-t disktype] [vnd_dev] image\n");
-       fprintf(stderr,
-           "       vnconfig -l [vnd_dev]\n");
-       fprintf(stderr,
+           "usage: vnconfig [-v] [-k | -K rounds [-S saltfile]] "
+           "[-t disktype] [vnd_dev] image\n"
+           "       vnconfig -l [vnd_dev]\n"
            "       vnconfig -u [-v] vnd_dev\n");
        exit(1);
 }