(grep-highlight-matches): Change default from

(grep-highlight-matches): Change default from
`t' to `auto-detect', and type from `boolean' to `choice'.
(grep-compute-defaults): Set `grep-highlight-matches' to `t'
if grep option `--color' is available, or to `nil' otherwise.
(grep-use-null-device): Remove redundant type `boolean' (`choice'
is already defined).