[Ltib] proxy testing updates

[Stuart Hughes]

I've checked in some changes to improve the handling of proxy servers.  

Here's what's changed:

In bin/Ltibutils.pm:

* $SIG{__DIE__} is locally set to 'DEFAULT' in get_file to allow
LWP::UserAgent to cycle through the returned challenge types.

* If http_proxy is not set, but proxy is set, the proxy is forced off
with not test being run.

* A message is printed to tell you that the proxy is being tested.

* If the proxy test fails, the status line is printed out to enable you
to try to figure out what went wrong.

In ltib: 

* There is a commented out: #use LWP::Debug qw(+); if you run into
problems with proxying, you can uncomment this to get some output from
LWP.

Regards, Stuart