Support a separate face for displaying the current line's number

Support a separate face for displaying the current line's number

* lisp/faces.el (line-number-current-line): New face.

* src/xdisp.c (syms_of_xdisp) <line-number-current-line>: New
symbol.
(try_window_id, try_cursor_movement): Disable these optimizations
when the line-number-current-line face is different from
line-number face.
(maybe_produce_line_number): Display the current line in the
line-number-current-line face, if it's different from line-number.