Initialize cl--gensym-counter to 0

Initialize cl--gensym-counter to 0

Previously it was initialized to a random value, which made it
harder to reproduce earlier Emacs runs.  The need for a random
value went away when Emacs introduced and used the #: syntax for
uninterned symbols (Bug#20862).
* doc/misc/cl.texi (Creating Symbols, Common Lisp Compatibility):
Document that cl--gensym-counter now starts with 0.
* lisp/emacs-lisp/cl-lib.el (cl--gensym-counter): Remove.
(cl--random-time): Move to near only remaining use.
* lisp/emacs-lisp/cl-macs.el (cl--gensym-counter): Initialize to 0.