maint: ftruncate is always available, even without gnulib

maint: ftruncate is always available, even without gnulib

Now that even MinGW provides ftruncate, we know that all
reasonable portability targets provide this function.
Remove the workaround code.  We nearly removed the gnulib
module three years ago:
  http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/9203
and it is now officially "obsolete".
* bootstrap.conf (gnulib_modules): Remove ftruncate.
* src/copy.c (copy_reg): Remove use of HAVE_FTRUNCATE and its
no-longer-used workaround code.
* src/truncate.c: Remove a comment about handling missing ftruncate.