install: allow options -D and -t to be used together

install: allow options -D and -t to be used together

* src/install.c (install_file_in_file_parents): Factor out the
creation of any parent directories into ...
(mkancesdirs_safe_wd): ... this new function.
(install_file_in_dir): Add the parameter 'mkdir_and_install', and
call the above new function if it evaluates to true.
(main): During parsing of the -t option, move the check whether
the target_directory exists down  after the option parsing loop,
and do not complain about stat(optarg,...) failing if -D was given.
Pass 'mkdir_and_install' to install_file_in_dir().
* doc/coreutils.texi (install invocation): Remove the (false)
restriction that -D would be ignored together with -t.  Instead,
clarify install's new bahavior.
Fix the node's reference in the top-level @direntry for consistency.
* src/install/basic-1.sh: Add tests for the now-allowed combination
of the -D and -t options.
* NEWS: Mention the improvement.