(byte-compile-warnings): Document `not'.

(byte-compile-warnings): Document `not'.
(byte-compile-warnings-safe-p): Handle `not'.
(byte-compile-warning-enabled-p, byte-compile-disable-warning)
(byte-compile-enable-warning): New functions.
(byte-compile-eval-before-compile)
(byte-compile-file-form-require): Use byte-compile-disable-warning.
(byte-compile-close-variables): Locally bind byte-compile-warnings,
but do not modify it.
(byte-compile-eval, byte-compile-obsolete)
(byte-compile-warn-about-unresolved-functions)
(byte-compile-file-form-defvar)
(byte-compile-file-form-custom-declare-variable)
(byte-compile-file-form-require)
(byte-compile-file-form-defmumble, byte-compile-lambda)
(byte-compile-form, byte-compile-normal-call)
(byte-compile-variable-ref, byte-compile-defvar)
(byte-compile-make-variable-buffer-local):
Use byte-compile-warning-enabled-p.