bsd-user/signal.c: sigset manipulation routines.

bsd-user/signal.c: sigset manipulation routines.

target_sigemptyset: resets a set to having no bits set
target_sigaddset:   adds a signal to a set
target_sigismember: returns true when signal is a member
host_to_target_sigset_internal: convert host sigset to target
host_to_target_sigset: convert host sigset to target
target_to_host_sigset_internal: convert target sigset to host
target_to_host_sigset: convert target sigset to host

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Kyle Evans <kevans@freebsd.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>