thread API & CPU session: accounting of CPU quota

In the init configuration one can configure the donation of CPU time via
'resource' tags that have the attribute 'name' set to "CPU" and the
attribute 'quantum' set to the percentage of CPU quota that init shall
donate. The pattern is the same as when donating RAM quota.

! <start name="test">
!   <resource name="CPU" quantum="75"/>
! </start>

This would cause init to try donating 75% of its CPU quota to the child
"test".  Init and core do not preserve CPU quota for their own
requirements by default as it is done with RAM quota.

The CPU quota that a process owns can be applied through the thread
constructor. The constructor has been enhanced by an argument that
indicates the percentage of the programs CPU quota that shall be granted
to the new thread. So 'Thread(33, "test")' would cause the backing CPU
session to try to grant 33% of the programs CPU quota to the thread
"test". By now, the CPU quota of a thread can't be altered after
construction. Constructing a thread with CPU quota 0 doesn't mean the
thread gets never scheduled but that the thread has no guaranty to receive
CPU time. Such threads have to live with excess CPU time.

Threads that already existed in the official repositories of Genode were
adapted in the way that they receive a quota of 0.

This commit also provides a run test 'cpu_quota' in base-hw (the only
kernel that applies the CPU-quota scheme currently). The test basically
runs three threads with different physical CPU quota. The threads simply
count for 30 seconds each and the test then checks wether the counter
values relate to the CPU-quota distribution.

fix #1275