Remove defining user variables via * in docstring.

Remove defining user variables via * in docstring.

* lisp/apropos.el (apropos-variable):
* lisp/files-x.el (read-file-local-variable):
* lisp/simple.el (set-variable):
* lisp/woman.el (woman-mini-help):
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Callers changed.

* lisp/custom.el (custom-variable-p): Return nil for non-symbol
arguments instead of signaling an error.
(user-variable-p): Obsolete alias for custom-variable-p.

* lisp/erc/erc.el (erc-cmd-SET): Call custom-variable-p instead of
user-variable-p.

* src/callint.c (Finteractive, Fcall_interactively):
* src/minibuf.c (Fread_variable): Callers changed.

* src/eval.c (Fuser_variable_p, user_variable_p_eh)
(lisp_indirect_variable): Functions deleted.
(Fdefvar): Caller changed.

* doc/lispref/commands.texi (Interactive Codes):
* doc/lispref/help.texi (Accessing Documentation):
* doc/lispref/minibuf.texi (High-Level Completion): Callers changed.

* doc/lispref/customize.texi (Variable Definitions): Remove user-variable-p.