patch 9.1.1456: comment plugin fails toggling if 'cms' contains \

patch 9.1.1456: comment plugin fails toggling if 'cms' contains \

Problem:  comment plugin fails toggling if 'cms' contains \
Solution: escape backslash (Maxim Kim)

groff could be commented using \" or \# and comment plugin fails to
uncomment such things.

NOTE: if newstyle comment would be introduced, e.g. .\#
for groff test would need to be adjusted.

closes: #17530

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>