dash: Cache the expanded prompt for editline

Previously, the prompt would be expanded every time editline called the
getprompt callback. I think the code may have been written assuming that
editline only calls getprompt once per prompt, but it may actually call
it many times, for instance every time you type backspace. This results
not only in slower editing from expanding complex prompts repeatedly, it
also consumes more and more stack memory each time getprompt is called.
This can be seen by setting PS1 to some command substitution, typing
many characters at the prompt, then holding backspace and observing
memory usage. Thankfully all this stack memory is freed between prompts
by the stackmark calls around el_gets.

This change causes prompt expansion to always happen in the setprompt
call, as it would when editline is disabled, and a cached copy of the
prompt is saved for getprompt to return every time editline calls it.
Since getprompt is no longer doing expansion, the stackmark calls
surrounding el_gets can be removed.

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