trim(8): add another safety net

It is quite easy make a mistake and run something like this:

	trim -f /dev/da0 -r rfile

This would trim the whole device then emit an error on non-existing file -r.

Add another check to prevent this while allowing this form still
for real object names beginning from dash:

	trim -f -- /dev/da0 -r rfile

MFC after:	1 week

usr.sbin/trim/trim.c

@@ -40,6 +40,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sysexits.h>
 #include <unistd.h>
 
@@ -104,6 +105,23 @@ main(int argc, char **argv)
 			/* NOTREACHED */
 		}
 
+	/*
+	 * Safety net: do not allow mistakes like
+	 *
+	 *	trim -f /dev/da0 -r rfile
+	 *
+	 * This would trim whole device then error on non-existing file -r.
+	 * Following check prevents this while allowing this form still:
+	 *
+	 *	trim -f -- /dev/da0 -r rfile
+	 */
+	
+	if (strcmp(argv[optind-1], "--") != 0) {
+		for (ch = optind; ch < argc; ch++)
+			if (argv[ch][0] == '-')
+				usage(name);
+	}
+
 	argv += optind;
 	argc -= optind;