fix timestamp handling in DBUS setActive and setActiveWithUser handlers
a bit optimize process_new()
improve system_flags_changed/u_proc.changed variables setting on flag creation or removal

+ u_flag.urgent

(boolean) If TRUE, adding/removing the flag to/from an entity (u_proc or
system flags) will tag the entity as changed, i.e. u_proc.changed or
system_flags_changed variable will be set.

Default value for new flags is 1 (TRUE).

+ functions that add/removes a flag to/from entity (process or system
flags) get extra argument to specify if the entity should be tag as
changed, overriding default behaviour determined by flag.urgent.
Same applies for DBUS methods.

+ flag.inherit causes all children to be marked as changed
set_active_pid(): flag active processes

flag properties:
	name = "active"
	priority = <active list position, 1 for topmost>
	inherit = 1

Whenever the process position is changed the flag is updated and new
iteration scheduled. This allows match active processes and their children
in mappings.

Running scheduler iteration is slow, but to schedule affected processes
only, serious refactoring of filtering and processing code in core.c.
scheduler: implement "placeholder" cgroups

Put processes only in children cgroups, never in the "placeholder" cgroup
itself.
If the process won't match any of child rule, the rule will not match.

This allows the process to be check against multiple cpu cgroups "trees"
without the need of specifying all conditions on root rules. This way we
can create trees with strong focus on dividing cpu.shares by arbitrary
process properties.