execute: also hook up ansi-seq-based terminal size determination with exec_context_de...

execute: also hook up ansi-seq-based terminal size determination with exec_context_determine_size()

And while we are at it, merge exec_context_determine_tty_size() +
exec_context_apply_tty_size().

Let's simplify things, and merge the two funcs, since the latter just
does one more call.

At the same time, let's make sure we actually allow passing separate
input/output fds.