Adjust formatting of grep and zgrep manual pages.

Adjust formatting of grep and zgrep manual pages.

grep(1) changes:
 - Pet mandoc & igor.
 - Stylize the text more with macros when appropriate.
 - Stylize equal signs in long options (e.g., "--color=auto") with
   the "Cm" macro as suggested by mdoc(7).
 - Add missing arguments to --exlude, --exclude-dir, --include and
   --include-dir.
 - Remove a duplicate entry for the --context flag.
 - Use a list in the EXAMPLES sections to make it easier to tell
   which paragraphs belong to which example.
 - Cross reference zgrep(1).

zgrep(1) changes:
 - Fix Nd.
 - Split synopsis into paragraphs for readability.
 - Cross reference bzip(1), grep(1) and xz(1).

Reviewed by:	bcr
Approved by:	mat (mentor)
Differential Revision:	https://reviews.freebsd.org/D16779