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', '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.

(Explicit clean-up of unused tracing directories and files is currently
not implemented.)