History, completion and bash -i work like bash

Bashkit now implements bash’s interactive builtins, with state kept per interpreter:

• history and fc under set -o history or bash -i, with HISTSIZE, HISTFILESIZE, histappend and HISTFILE in the VFS. fc reruns commands but opens no editor.
• complete, compgen and compopt with all actions.
• bind with bash’s default emacs bindings.
• ${x@P} prompt decoding.
• A bash -i child shell with PS1, PS2 and PS4 defaults, ~/.bashrc or --rcfile, and --norc.

The Oils spec pass rate went from 89.0% to 92.8%.

See the compatibility scorecard and #2648.