This will be applied to the pty's termios settings at creation time,
superseding the default settings uxpty has always used. It works by
including the new sshttymodes.h with TTYMODES_LOCAL_ONLY defined, so
that modes not supported by a particular Unix system are automatically
quietly ignored.
Of course, a struct ssh_ttymodes always has the option of representing
"please make no change to the defaults", and of course, that's
precisely what is done by the one that pty_init constructs for clients
that aren't calling pty_backend_create directly.
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