one-seat-desktop: better messages about freezing/thawing cgroups
remove unused code for CGroup cleaning from ulatencyd.c
improve reaction on active session switch

- better logging
- make sure full iteration is run before next xwatch poll, if we allow
xwatch poll on server running some frozen application it may lock
core.lua: add string:rtrim(), fix CGroup:get_value()
one_seat_desktop rules: suppress warnings when swappiness and use_hierarchy cgroups parameters can't be written

These errors are common when resuming suspended ulatencyd without prior
cgroups clean up, memory.use_hierarchy can't be changed if enabled for
parent cgroup, and if so, swappiness can't be changed either.
ulatencyd: better log messages on daemon start
lower the log level in proc:set_block_scheduler()
core.lua: suppress warning if the task that can't be added to cgroup is already dead

+ new method ulatency.is_pid_alive(pid)
Check `/proc` subsystem and return TRUE if process/task with `pid` exists.
Isolate lua filter: log the process isolation only if flag.threshold > 1

Assuming isolations with threshold 1, i.e. those without
own isolation cgroup and not surviving ulatencyd shutdown,
are internally used by ulatencyd, i.e. cgroup release agent script, that
is run quiet frequently, stop bother user with these.
io.lua rule: log only block devices which parameters were really changed

This prevents useless logging for e.g. loopbacks.
ulatencyd.conf: raise io.threshold from 500 to 3000, 500 was IMHO too low