CVE-2026-59845: check the fork() return code when starting a ProxyCommand
so a failed fork is not stored as pid -1 and later killed as a process group
CVE-2026-59843: reject a maximum packet size of 0 in SSH2_MSG_CHANNEL_OPEN and
SSH2_MSG_CHANNEL_OPEN_CONFIRMATION and clamp channel writes on remote_maxpacket
directly, avoiding the non-progressing loop in channel_write_common()
CVE-2026-59848: track outstanding SFTP request IDs and reject responses
carrying an unknown ID instead of queueing them indefinitely