install-sh: be stricter in catching invalid usages

install-sh: be stricter in catching invalid usages

Such usages (which are rejected by GNU install as well) are:

  - options -d and -t used together;

  - argument passed to option -t must be a directory;

  - if there are two or more SOURCEFILE arguments, the
    DESTINATION argument must be a directory.

Note that we still allow the use of options -d and -T together, by
making -d take the precedence; this is for compatibility with GNU
install.

This change fixes, among other things, automake bug#15376.

* lib/install-sh: Adjust.
* t/install-sh-unittests.sh: Enhance.
* NEWS: Update.
* THANKS: Add reporter of bug#15376.

Helped-by: Tobias Hansen <thansen@debian.org>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>