Rewrote the local minor mode so that it can be

Rewrote the local minor mode so that it can be
sticky as well and made sticky the default.  Reimplemented the
global minor mode.  Updated the commentary section to document
these changes.
(hl-line-sticky-flag): New user option.
(hl-line-overlay): Made it buffer-local and gave it a docstring.
(global-hl-line-overlay): New variable.
(hl-line-mode): Rewritten to use `hl-line-sticky-flag'.
(hl-line-highlight): Rewritten to use `hl-line-sticky-flag'.
(hl-line-unhighlight): Updated docstring.
(global-hl-line-mode): Implemented directly so that is does not
depend on `hl-line-mode' any more.
(global-hl-line-highlight, global-hl-line-unhighlight): New
functions.