`PtyMaster` missing `Into<OwnedFd>`

[Darksonn]

An implementation of AsFd was added previously in #2354, but this is not enough to safely convert the PtyMaster into e.g. a File. There should be a conversion from PtyMaster to OwnedFd to allow this.

[SteveLauC]

Yeah, we should add this, std implements this for all the owned fd types