Allow Apache2 binary to be configured

Required for Mac OS X support

e.g.

[default:apache2]
binary=/usr/sbin/httpd
Warn the user when a server has no configured host
Use load average on Mac OS to throttle Apache2

This isn't as good as the Linux implementation, but it's better
than nothing.

Weaknesses include:

- Load isn't a good indicator of memory pressure - killing a
  server may well cause us to increase load when we attempt
  to respawn it

- We are using the same values for reaping as creation, so
  we may kill new connections instead of reaping old ones -
  which is undesirable.