doc: homologize ch{con,grp,mod,own} option descriptions

doc: homologize ch{con,grp,mod,own} option descriptions

* src/chgrp.c (usage): Group associated options together,
to aid users.  Also minimize the differences between
individual messages across these four commands, to
aid translators.
* src/chmod.c: Likewise.
* src/chown.c: Likewise.
* src/chcon.c (usage): Likewise.  Document the
--dereference option.

Suggested by Paul Eggert and Jari Aalto