bug#70833: 29.2.50; project-current: allow control of the prompt string

[Spencer Baugh]

It would be nice to be able to control the prompt string that
project-current uses when prompting for a project.  Ideally, by passing
a string for the MAYBE-PROMPT argument.

This is useful when using project-current in functions which should
operate on the current project normally, but which might be run outside
any project and in that case will prompt for a project.  The user might
be uncertain about what the project they'll input will be used for,
exactly.  It's useful for the prompt to remind them.

The prompt could be passed down as an argument to project-prompter, or
bound in a defvar for compatibility - either seems fine to me.