doc: remove extraneous periods from --help output

doc: remove extraneous periods from --help output

* src/join.c (usage): Mention "fields" rather than repeating "line"
so that it's more obvious that the fields are still parsed, and
thus -o is still honored for headers.  Also remove an extraneous
'.' reported by Stéphane Raimbault.
* src/base64.c (usage): Remove extraneous blank line and order
the options alphabetically.  Also remove an extraneous '.'
* src/chown.c (usage): Remove extraneous '.'
* src/cp.c (usage): Likewise.
* src/mktemp.c (usage): Likewise.
* src/pr.c (usage): Likewise.
* src/stat.c (usage): Likewise.
* src/uniq.c (usage): Likewise.