fish: edit: factor out download and reupload phases

Share some code between edit_file_editor and edit_file_perl; mostly code
motion, with no actual behaviour change.
fish: edit: centralize the EDITOR handling

Allow null as value for the editor parameter of edit_file_editor, which
will then get it from the EDITOR envvar (falling back on vi).

This is basically code motion from the two edit_file_editor users to it.
test-tool: stop printing the FEBOOTSTRAP_* envvars

Stop printing the FEBOOTSTRAP_* environment variables, since they are
not used anymore.