Allow psql's other uses of simple_prompt() to be interrupted by ^C.
This fills in the work left un-done by
5f1148224. \prompt can
be canceled out of now, and so can password prompts issued during
\connect. (We don't need to do anything for password prompts
issued during startup, because we aren't yet trapping SIGINT
at that point.)
Nathan Bossart
Discussion: https://postgr.es/m/747443.
1635536754@sss.pgh.pa.us