Apply non-user themes only when asked

Apply non-user themes only when asked

Theme settings now generally aren't actually applied until a call to
`enable-theme-, either one made explicitly or implicitly through
`load-theme' with NO-ENABLE nil.  This change has the effect of not
applying theme changes just because we load a lisp file containing a
theme specification.  The previous behavior is preserved for the
special case of the `user' theme, which is frequently used for
ad-hoc customization.

* lisp/cus-face.el (custom-theme-set-faces): Call
`custom--should-apply-setting' to decide whether to apply
a setting.

* lisp/custom.el (custom--should-apply-setting): New function.
(custom--inhibit-theme-enable): Add `apply-only-user' option;
default to it.
(custom-push-theme, custom-theme-set-variables): Call
`custom--should-apply-setting' to decide whether to apply
a setting.