Fix self lockout on VDE socket

[Janrupf]

SIMH seems to somewhat quietly lock itself out of its own VDE socket because it sets the permission bits to all zero. I found some resources saying SIMH needs to run as root to use VDE, which seems to also stem from this problem. Now it works without running as root

[pmetzger]

Weird because this effect seems to contradict the VDE documentation but testing shows the patch is indeed correct. I applied it to ZIMH in this commit; I have no control over the SIMH repository. I used mode 0600 as it appears there was no need to give the socket an execute bit.

[Janrupf]

Ah whoops, you are right on the permission bits. I think I just had the 7 there for testing and then forgot to change it. Fixed that now

[Janrupf]

Regarding the VDE documentation, @pmetzger where would you say it contradicts it? I think its worded a bit weirdly with "[...] has been left for backwards compatibility", but it only explicitly mentions that the open args can be set to NULL (in which case VDE would apply mode 0700, interestingly enough also including the executable bit...), but not that they can be zeroed or are ignored.

I'm considering making a PR at VDE to maybe ignore the mode if its set to zero since I don't see how that would ever be useful, but arguably VDE shouldn't work around bugs in applications.

[pmetzger]

The documentation seemed to indicate to me that zero meant defaults would be applied.

Anyway, it's rare that I've seen such a simple fix completely correct a problem. Thank you for bringing it to everyone's attention. I hope SIMH also applies it.