* cmd/clef, signer/core: use better terminal input for passwords, make it possible to avoid boot-up warning

* all: move commonly used prompter to isolated (small) package

* cmd/clef: Add new --acceptWarn to clef README

* cmd/clef: rename flag 'acceptWarn' to 'suppress-bootwarn'
Co-authored-by: ligi <ligi@ligi.de>