Do not reopen temporary files

Do not reopen temporary files

The current callers of mkostemp close the file descriptor and then
re-open it with fopen.  It seemed better to me to continue to use the
already-opened file descriptor, so this patch rearranges the code a
little in order to do so.  It takes care to ensure that the files are
only unlinked after the file descriptor in question is closed, as
before.

gdb/ChangeLog
2018-10-27  Tom Tromey  <tom@tromey.com>

	* unittests/scoped_fd-selftests.c (test_to_file): New function.
	(run_tests): Call test_to_file.
	* dwarf-index-write.c (write_psymtabs_to_index): Do not reopen
	temporary files.
	* common/scoped_fd.h (scoped_fd::to_file): New method.