Move string utility from rpc_args.h to string.h

The buffer handling implemented by 'Rpc_in_buffer' is handy in many
situation in addition to the use of strings as RPC arguments. By moving
the functionality to util/string.h, we make it more easily available.
base: Add tracing support to CPU session interface
base: User-level tracing support
core: Add tracing support to CPU service
TRACE service interface and implementation
XXX call tracing hooks from init (just for testing)