Mention signal fixes in NEWS

[Andrew Cagney]

+Many generic problems with GDB's signal handling code have been fixed.
+These include: support for sigaltstack and sigaction;


I'd prefer that we spell out what this ``support for sigaltstack and
sigaction'' means, in user-level terms.  That is, what exactly didn't
work before and now does, when sigaltstack and sigaction are involved?
... include: backtraces through non-contigious stacks (see 
sigaltstack(2)); recognition of sa_sigaction signal trampolines (see 
sigaction(2)); ...

+Note that kernel and architecture specific limitations may still
+restrict this functionality (ref PR gdb/1639).


How about being more explicit here?  What configurations have this
new support, and which don't?  Can we list at least the more popular
platforms in each category?
That gets complicated.  Since some features only work on specific 
patched kernels.  However we can probably safely state that the features 
do work on ``i386 GNU/Linux systems that include a 2.4 kernel''.

Andrew