bsd-user: Add os-file.h with chflagsat, close_range, and more

Add FreeBSD-specific file operation shims: chflagsat, close_range,
copy_file_range, and __specialfd. Also add the safe_copy_file_range and
safe_ppoll syscall wrappers and the target_specialfd_eventfd type
definitions.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Assisted-by: Claude Opus 4.6 (1M context)
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>