pwconv and grpconv: rewind after deleting an entry

Otherwise our spw_next() will cause us to skip an entry.
Ideally we'd be able to do an swp_rewind(1), but I don't
see a helper for this.

Closes #60

Signed-off-by: Serge Hallyn <shallyn@cisco.com>

src/grpconv.c

@@ -198,6 +198,7 @@ int main (int argc, char **argv)
 			         Prog, sg->sg_name, sgr_dbname ());
 			fail_exit (3);
 		}
+		(void) sgr_rewind ();
 	}
 
 	/*