xinstall: Do not allow -l and -s together

Cannot strip the target if creating a link.

Reviewed by:	des
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57398

usr.bin/xinstall/tests/install_test.sh

@@ -27,8 +27,11 @@
 
 atf_test_case incompatible_opts
 incompatible_opts_body() {
+	printf 'test\n123\r456\r\n789\0z' >testf
 	atf_check -s not-exit:0 -e match:"specified together" \
 	    install -s -d dir1
+	atf_check -s not-exit:0 -e match:"specified together" \
+	    install -s -l s testf copyf
 }
 
 atf_test_case copy_to_empty

usr.bin/xinstall/xinstall.c

@@ -293,6 +293,12 @@ main(int argc, char *argv[])
 		usage();
 	}
 
+	/* Cannot strip if creating a link. */
+	if (dostrip && dolink) {
+		warnx("-l and -s may not be specified together");
+		usage();
+	}
+
 	/*
 	 * Default permissions based on whether we're a directory or not, since
 	 * an +X may mean that we need to set the execute bit.