cmake-mode.el: Fix extracting keyword at point in cmake-help

Since commit v3.1.0-rc2~1^2~1 (cmake-mode.el: syntax of '_' should be
treated as symbol, 2014-11-12) the 'word-at-point' function does not
extract the whole keyword anymore if it contains an '_', because
'forward-word' stops at '_'.  Use 'symbol-at-point' to extract a whole
keyword even if there is an '_'.