add my_[l]stat_flags(); make my_[l]stat() mathoms

add my_[l]stat_flags(); make my_[l]stat() mathoms

my_stat() and my_lstat() call get magic on the stack arg, so create _flags()
variants that allow us to control this. (I can't just change the signature
or the mg_get() behaviour since my_[l]stat() are listed as being in the
public API, even though they're undocumented.)