gdb, gdbserver: make status_to_str display the signal name

gdb, gdbserver: make status_to_str display the signal name

I was looking at some "set debug lin-lwp" logs, and saw that a thread
received the "Child exited" signal.  It took me a moment to realize that
this was SIGCHLD.  I then thought that it would be nice for
status_to_str to show the signal name (SIGCHLD) in addition to the
description "Child exited", since people are much more used to referring
to signals using their names.

Fortunately, libiberty contains a handy function to get the signal name
from the signal number, strsigno, use that.

The output of "set debug lin-lwp" now looks like:

    [linux-nat] linux_nat_wait_1: waitpid 1209631 received SIGTRAP - Trace/breakpoint trap (stopped)

gdb/ChangeLog:

	* nat/linux-waitpid.c (status_to_str): Show signal name.

Change-Id: I8ad9b1e744dd64461fd87b08d5c29f9ef97c4691