Fix some section name typos in various manual pages.

While here, fix some other minor issues like starting new sentences
on new lines etc.

Use standard section name 'AUTHORS' in several manual pages.

kernel/ddb: Add a DB_COMMAND_FLAG() macro to set the flag, too.

unquote section names in man pages

While there, make them upper case, try to simplify and
consolidate, zap comments that nobody will ever see again
and tweak one AUTHORS instance with '.An -nosplit'.

start adding .Mt to email addresses in man pages

This covers share/man/man[1579]. It also consolidates multiple lines
into one, removes spurious double quotes in names or emails,
and changes punctua

start adding .Mt to email addresses in man pages

This covers share/man/man[1579]. It also consolidates multiple lines
into one, removes spurious double quotes in names or emails,
and changes punctuation where appropriate. The target style is:
.An Name Aq Mt name@somewhere.tdl

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Add a DB_COMMAND(9) manual page (adapted from FreeBSD).