Search recursively in gzipped files. (Bug#4982)

Search recursively in gzipped files.  (Bug#4982)

(grep-highlight-matches): Add new options
`always' and `auto'.  Doc fix.
(grep-process-setup): Check `grep-highlight-matches' for
`auto-detect' to determine the need to compute grep defaults.
Move Windows/DOS specific --colors settings handling
to `grep-compute-defaults'.  Check `grep-highlight-matches'
to get the value of "--color=".
(grep-compute-defaults): Compute `grep-highlight-matches' when it
has the value `auto-detect'.  Move Windows/DOS specific settings
from `grep-process-setup'.
(zrgrep): New command with alias `rzgrep'.