This closes patch:

[ 960406 ] unblock signals in threads

although the changes do not correspond exactly to any patch attached to
that report.

Non-main threads no longer have all signals masked.

A different interface to readline is used.

The handling of signals inside calls to PyOS_Readline is now rather
different.

These changes are all a bit scary!  Review and cross-platform testing
much appreciated.
diff --git a/Misc/ACKS b/Misc/ACKS
index b6e4da4..e67b754 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -327,6 +327,7 @@
 Andrew Kuchling
 Vladimir Kushnir
 Cameron Laird
+Andrew Langmead
 Detlef Lannert
 Soren Larsen
 Piers Lauder