fix for Exporter's $SIG{__WARN__} handler

fix for Exporter's $SIG{__WARN__} handler

Exporter's $SIG{__WARN__} handler is too zealous.  It fails here:

    $ cat t
    require Carp;
    package Foo;
	@ISA = qw(Exporter);
	@EXPORT_OK = @EXPORT_FAIL = qw(foo);
	sub export_fail {
	    my $self = shift;
	    Carp::carp("carp from export_fail");
	    return ();
	}
    package main;
	import Foo 'foo';
    $ perl5.00393 t
    carp from export_fail at t line 16
     at t line 16

This is because the carp() inside export_fail triggers Exporter's
__WARN__ handler which calls carp() a second time which adds the second
" at t line 16".

I was surprised to learn that carp() adds its string to an error message
which ends with a newline, I thought it followed warn()'s lead on that.
I checked 5.003 and it was the same there, too, though.

Also, shouldn't Exporter's __WARN__ handler increment $Carp::CarpLevel
rather than setting it to 1?  I didn't include that in the patch because
I'm not sure, but it seems the right thing to do to me.

p5p-msgid: 2282.858296451@eeyore.ibcinc.com