Added 'special' descriptors, and fixed bugs in fd/service indexing

There are now 6 default named file descriptors:
  * null
  * stdin
  * stdout
  * stderr
  * control.event
  * control.cmd

In the process, I discovered and fixed a number of bugs in the comparison
functions for the indexes of services and fds, and some bugs in the
iterators, and a bug in the fd pointer array allocator.

Implements half of #8
Merge branch 'master' of github.com:silverdirk/daemonproxy