time to clean up linux-nat.c debugging messages

[Doug Evans]

Turning on debugging messages of linux-nat.c, for example, results
in messages like these:

LLW: Candidate event Trace/breakpoint trap (stopped) in Thread 0x7ffff7fd7700 (LWP 14143).

and

SC:  kill LWP 14146 **<SIGSTOP>**

The first is from linux_nat_wait_1 and the second is from stop_callback.

I'm guessing prefixes like LLW and SC had some original use,
but they get in the way of mapping the output back to the source.
That's not good.

Anyone mind if I replace all these prefixes with the name of the
function they come from (which AIUI is pretty standard)?