use PATH_MAX

Include <limits.h> and use the PATH_MAX macro to define the length of
buffers containing paths to make it constistent. (It's not supposed to
fit all possible paths.)

ntp_signd.c

@@ -129,7 +129,7 @@ close_socket(void)
 static int
 open_socket(void)
 {
-  char path[1024];
+  char path[PATH_MAX];
 
   if (sock_fd != INVALID_SOCK_FD)
     return 1;