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that ends a comment, when moving forward.
(Fbackward_prefix_chars): Return immediately if point is at BEGV.

(prev_char_comend_first): New function.
(back_comment): Use that.  Carefully update syntax table position
for each character that is fetched.
(Fforward_comment): Likewise.
(scan_lists): Likewise.
(prev_char_comstart_first): New function.