honor rpath $ORIGIN for ldd/ldso command with program in working dir

the rpath fixup code assumed any module's name field would contain at
least one slash, an invariant which is usually met but not in the case
of a main executable loaded from the current working directory by
running ldd or ldso as a command. it would be possible to make this
invariant always hold, but it has a higher runtime allocation cost and
does not seem useful elsewhere, so just patch things up in fixup_rpath
instead.

ldso/dynlink.c

@@ -807,7 +807,16 @@ static int fixup_rpath(struct dso *p, char *buf, size_t buf_size)
 		origin = p->name;
 	}
 	t = strrchr(origin, '/');
-	l = t ? t-origin : 0;
+	if (t) {
+		l = t-origin;
+	} else {
+		/* Normally p->name will always be an absolute or relative
+		 * pathname containing at least one '/' character, but in the
+		 * case where ldso was invoked as a command to execute a
+		 * program in the working directory, app.name may not. Fix. */
+		origin = ".";
+		l = 1;
+	}
 	p->rpath = malloc(strlen(p->rpath_orig) + n*l + 1);
 	if (!p->rpath) return -1;