chimera: fix behavior on create of '.(fset)' files

the fset/pset files are special commands and we
do not allow to create them. Nevertheless, current
behavior is to throw IOError.

With this change will will throw Exists which will force client
to fall-back to regular open-for-write.

Fixes fset over NFSv4.1

Acked-by: Albert Rossi
Acked-by: Paul Millar
Target: master, 2.8, 2.7, 2.6
Require-book: no
Require-notes: no