maint: s/strncpy/memcpy/, when equivalent

* src/output.c (output_skeleton): Use memcpy, not strncpy,
since the source is known to fit in the destination buffer.
* src/parse-gram.y (%skeleton): Likewise.

src/parse-gram.y

@@ -322,7 +322,7 @@ prologue_declaration:
             xmalloc (dir_length + 1 + strlen (skeleton_user) + 1);
           if (dir_length > 0)
             {
-              strncpy (skeleton_build, current_file, dir_length);
+              memcpy (skeleton_build, current_file, dir_length);
               skeleton_build[dir_length++] = '/';
             }
           strcpy (skeleton_build + dir_length, skeleton_user);