pid1: add env var to override default mount rate limit burst
    
    I am hitting the rate limit on a busy system with low resources, and
    it stalls the boot process which is Very Bad (TM).
    
    (cherry picked from commit 24a4542cfa674ee80b54afcc223f2490a011966b)
    
    Included in the backport because it's a workaround for an issue introduced
    in v249.