upstreaming debian's xdm patches

[Julien Cristau]

I've now pushed these series, except for:

> Branden Robinson (7):
>       add support for logfile rotation
> 
> Julien Cristau (2):
>       Add timestamping to logging functions
>       Append to the log file instead of truncating it
> 
The log handling because of the caveat I mentioned in a followup
(rotating the xdm log means X stderr goes to /dev/null, and I'm not sure
if that's acceptable?  other opinions appreciated), and the timestamping
which needs to be reworked for signal safety.

Thanks again for the review.

Cheers,
Julien