New NOXSESSIONERRORS env variable check in Xsession

Currently, there is no way to disable .xsession-errors file. If you make it a symlink to /dev/null Xsession will write the log into /tmp and won't even remove file after logging out. But there are users who don't want the file written at all, check this for just one example: https://askubuntu.com/q/93718/20275 If you think that .xsession-errors serves some useful role I can gather arguments about:

1. Errors and warnings from different processes there just pollute search results because they are being published in all sorts of unrelated context.
2. Most of the bugs opened about warnings in there are left without a reaction.

From these premises I make assumption that a way to opt-in for disabling .xsession-errors is beneficial. This way users who will opt-in will have more chances of posting only relevant info about the problem not polluted with output from all processes.

I will write docs if devs are open to this change. Otherwise, I'll have to apply the patch locally.