Block signals except during the event loop. Closes:#692233.

Block signals except during the event loop.  Closes:#692233.

Previously we would interrupt whatever we were doing, willy-nilly.
Now we wait until the event loop is reentered.  Symptoms could include
locking up the server with a grab, and/or spinning on the cpu.

This patch is not very portable (it uses ppoll) but I think it will
work on all Debian architectures.

In the longer term we should update to new upstream, where the last
commits were in 2013.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>