bug#63496: process-attributes returns incorrect start, etime, and pcpu attributes when run inside a lxc container

[Scott Zimmermann]

in src/sysdep.c line 3475:
Lisp_Object boot = Ftime_subtract (now, uptime);

it calculates the boot time for linux based on the uptime. Inside lxc 
containers, /proc/uptime is the uptime of the container rather than of 
the host system. However in /proc/[PID]/stat, starttime is the time 
since system boot, of the host system, not since the container started. 
This leads to emacs computing incorrect values for start, etime, and 
pcpu for processes.

I think it should get the boot time from "btime" in /proc/stat, this is 
what ps does, which shows correct times for processes inside the 
container. If this sounds like an appropriate fix I'm happy to write and 
test a patch for it.