support setting No_COW on Perl <5.22

support setting No_COW on Perl <5.22

fileno(DIRHANDLE) only works on Perl 5.22+, so we need to use
dirfd(3) ourselves from Inline::C (or rely on chattr(1) being
installed).

While we're at it, rename `set_nodatacow' to `nodatacow_fd'
for consistency with `nodatacow_dir'.