bash: don't touch the promptvars option

Merged Johann Dreo requested to merge github/fork/cmm/leave-promptvars-alone into master

Created by: cmm

Rationale:

  • In general, it is not proper for liquidprompt to change shell options that it does not absolutely have to change.

  • LP_PS1_{PREFIX,POSTFIX} are a thing, and are in fact an effective means to combine liquidprompt with other prompt-touching logics (such as emacs-vterm). So it is not good to prevent those other logics from actually working.

This commit restores the bash version of __lp_escape to what it did prior to 62f02708 (escaping dollar signs in addition to backslashes) and removes all interference with the promptvars shell option.

NOTE: I don't know much about shells other than bash, so left the parallel logic for zsh as is.

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