Detect auto-generated mail correctly when there are multiple Precedence headers

Example in the wild of multiple Precedence headers includes:

    Precedence: never-bounce
    Precedence: bulk

When $head->get() is called in scalar context, only the first value is
returned.  Instead, return true if any of the multiple possible values
match.
Correct documentation of CheckForAutoGenerated() to include all headers
Merge 4.2/multiple-precedence into 4.2-trunk