core: change policy incrementation to prefix op

Using prefix incrementation makes sure a policy with id == 0 is
invalid.
os: initial version of trace_fs

The trace_fs server provides access to a Trace_session by using a
File_system_session as frontend.

Each trace subject is represented by a directory ('thread_name.subject')
that contains specific files ('active', 'cleanup', 'enable', 'events',
'max_size' and 'policy'), which are used to control the tracing process
of the thread as well as storing the content of its trace buffer.

The tracing of a thread is only activated if there is a valid policy
installed and the intend to trace the subject was made clear by writing
'1' to the 'enable' file.

The tracing of a thread may be deactived by writing a '0' to the
'enable' file.

A policy may be changed by overwriting the currently used one. In this
case the old policy is replaced by the new policy and is automatically
utilize.

Writing a value to the 'max_size' file changes the appointed size of the
trace buffer. This value is only evaluted by reactivating the tracing
process.

The content of the trace buffer may be accessed by reading from the
'events' file. Throughout all tracing session new trace events are
appended to this file.

Nodes of UNTRACED subjects are kept as long as they do not change their
tracing state to DEAD. In this case all nodes are removed from the
file system. Subjects that were traced before and are now UNTRACED will
only be removed by writing '1' to the 'cleanup' file - even if they
are DEAD by now.

To use the trace_fs a config similar to the following may be used:

! <start name="trace_fs">
! 	<resource name="RAM" quantum="128M"/>
! 	<provides><service name="File_system"/></provides>
! 	<config>
! 		<policy label="noux -> trace" interval="1000" subject_limit="512" trace_quota="64M" parent_levels="0" />
! 	</config>
! </start>

'interval' sets the periode in which the Trace_session is polled. The
time is given in milliseconds.
'subject_limit' speficies how many trace subject should by acquired at
most when the Trace_session is polled.
'trace_quota' is the amount of quota the trace_fs should use for the
Trace_session connection. The remaing amount of RAM quota will be used
for the actual nodes of the file system and the 'policy' as well as the
'events' files.