Merge pull request #479 from hintjens/master

Tiny fixups
zdir now resistant to all file types

A directory full of symlink caused a zlist full of NULL. When the comparison
hook was called, no check was performed on the pointer's validity. Now it
doesn't segfault.
zlist now doesn't accept NULL items.

NULL seems to have a special meaning (e.g. end of list), so it is useless
and misleading to store it.
Merge pull request #481 from jfsmig/JFS-zdir-20140518

zdir now resistant to all file types
Merge pull request #482 from jfsmig/JFS-zlist-20140518

zlist now doesn't accept NULL items.
Merge pull request #480 from hintjens/master

Problem: process hangs on exit if there are open sockets
Merge pull request #483 from hintjens/master

Improvements to support zactor/zsock
Merge pull request #484 from hintjens/master

Fixed memory leak in socket leak detection
Problem: signal handling was broken

Ctrl-C handler wasn't being properly installed. This patch fixes
that.

Fixes #485.