("quotearg.h"): New include.

("quotearg.h"): New include.
(usage): Now has __attribute__ ((noreturn)).
(confirm): Report errno if we can't open tty.
(confirm, decode_options):
Quote arbitrary strings in diagnostics.
(OVERWRITE_OPTION): New constant.
(long_options, usage, decode_options): New --overwrite option.
(decode_options): --keep-old-files, --overwrite, and --unlink-first
are now mutually exclusive.
Don't assume that gettext preserves errno.
(main): Set default quoting style to escape_quoting_style.