bsd-user: Implement shmat(2) and shmdt(2)

Use `WITH_MMAP_LOCK_GUARD` instead of mmap_lock() and mmap_unlock(),
to match linux-user implementation, according to the following commits:

69fa2708 linux-user: Use WITH_MMAP_LOCK_GUARD in target_{shmat,shmdt}
ceda5688 linux-user: Fix shmdt

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Message-Id: <20230925182709.4834-23-kariem.taha2.7@gmail.com>