lib/args.c: add quotes.

This failure was a little hard to parse:

  tar: Unknown option mode (see "tar --help")

This would have been clearer:

  tar: Unknown option 'mode' (see "tar --help")

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/args.c b/lib/args.c
index b0bede19..89c82ca7 100644
--- a/lib/args.c
+++ b/lib/args.c
@@ -137,7 +137,7 @@ static int gotflag(struct getoptflagstate *gof, struct opts *opt)
   // Did we recognize this option?
   if (!opt) {
     if (gof->noerror) return 1;
-    help_exit("Unknown option %s", gof->arg);
+    help_exit("Unknown option '%s'", gof->arg);
   }
 
   // Might enabling this switch off something else?