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