Only print the errno string in case sysctl(3) does not file with ENOENT

This reduces the noise in error reporing from sysctl(8):

Before:
$ sysctl bla=something
sysctl: unknown oid 'bla': No such file or directory

After:
$ sysctl bla=something
sysctl: unknown oid 'bla'

MFC after:	1 week
Sponsored by:	Gandi.net

1 files changed, 5 insertions, 1 deletions

diff --git a/sbin/sysctl/sysctl.c b/sbin/sysctl/sysctl.c
index 8d67644..62f12b5 100644
--- a/sbin/sysctl/sysctl.c
+++ b/sbin/sysctl/sysctl.c
@@ -276,7 +276,11 @@ parse(const char *string, int lineno)
 		if (qflag)
 			return (1);
 		else {
-			warn("unknown oid '%s'%s", bufp, line);
+			if (errno == ENOENT) {
+				warnx("unknown oid '%s'%s", bufp, line);
+			} else {
+				warn("unknown oid '%s'%s", bufp, line);
+			}
 			return (1);
 		}
 	}