load-fragment: properly parse Nice= value

Hello,

     i`ve been using systemd for a while now, and found out that when
using NICE parameter for .service files the varible is not set correctly.
     i`ve found the problem in file *load-fragment.c* function
*config_parse_nice*        variable /*c->nice_set = false;*/  should be
/*c->nice_set = true;*/
     Problem is alsom manifesting on v17 but did not upgrade yet ...

src/load-fragment.c

@@ -306,7 +306,7 @@ static int config_parse_nice(
         }
 
         c->nice = priority;
-        c->nice_set = false;
+        c->nice_set = true;
 
         return 0;
 }