df(1): Allow duplicate -l flags gracefully

Rather than producing a misleading error message when duplicate -l flags are
provided to df(1), simply ignore extra flags and proceed as if only one was
specified.  This seems most reasonable given the usage for -l:

     -l      Only display information about locally-mounted file systems.

l and t flags still conflict, as before.

PR:		208169
Reported by:	by at reorigin.com
Reviewed by:	allanjude

bin/df/df.c

@@ -166,6 +166,9 @@ main(int argc, char *argv[])
 			hflag = 0;
 			break;
 		case 'l':
+			/* Ignore duplicate -l */
+			if (lflag)
+				break;
 			if (vfslist != NULL)
 				xo_errx(1, "-l and -t are mutually exclusive.");
 			vfslist = makevfslist(makenetvfslist());