The capsicum signal stuff is new with FreeBSD 14, rev 1400026, so only
define QEMU_SI_CAPSICUM there. Only copy _capsicum when QEMU_SI_CAPSICUM
is defined. Default to no info being passed for signals we make no guess
about.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>