The function that does the main pty setup is now called
pty_backend_create(), and has an API better suited to uxpty in
particular than the standard backend_init() virtual constructor. It
leaves off a load of standard parameters to backend_init() which
aren't really relevant to this backend, and it adds the 'argv'
parameter to pass in a split-up command line, which is unique to it.
The old creation function still exists, as a tiny wrapper that calls
the new pty_backend_create. And that version still gets the argv
parameter from the process-global variable pty_argv[], so the call
sites in pterm haven't had to change for this.
This will make it possible to instantiate a pty backend directly from
the SSH server code, without having to do anything really excessively
cumbersome to pass in a subcommand in the form of pre-split argv. (And
I'll add a few more specialist parameters to the new function shortly.)
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