Revert exclusion of makedev from buildd chroots, as it turned out not to
be the problem. Instead, fix up /dev/ptmx to be a character device node
rather than a symlink to /dev/pts/ptmx, in line with the discussion in
https://bugs.debian.org/817236; I think this is safer than
cherry-picking the fix to debootstrap at this point in a stable release
cycle.
LP: #1844504
