Add Free Documentation License. New section

"Overwrite Old Files", and revamp that section to make it easier to
follow.  "tar" -> "GNU tar" where appropriate.  Migrate getdate
documentation into getdate.texi.  Fix several minor typos.  Describe
TAR_OPTIONS.  Describe incompatibility between incremental backups and
--atime-preserve.  Describe incompatibility between --verify and other
options.  Mention that tar normally removes symbolic links rather than
following them, when extracting a file of the same name.