devpts: fix devpts mount behavior

This is the restart of an old conversation that started out with Serge noting that there are problems taking advantage of the devpts newinstance mount option. As the conversation progressed it was suggested that the devpts newinstance mount option should just be removed if possible. Then the conversation floundered upon what to do about /dev/ptmx. Making /dev/ptmx a symlink sounds simple, but in practice no one could actually figure out how to make it happen.

I was playing with this recently and it is not at all hard or complicated to have opens of ptmx redirect themselves to pts/ptmx.

So I propose we solve this issue by adding a little magic to /dev/ptmx and making the devpts newinstance mount option historical.

I my limited testing the code just works.

I assume that since devpts is essentially serial this should go through Greg's tree?

Eric