Better description of the installation steps:

Better description of the installation steps:
- document questions in proper order: hostname before network configuration,
  DNS only if network is configured, sshd and ntpd, aperture only on
  machines which need it, serial console only on machines which need it.
  This is done by splitting the end of OpenBSDCommonURL in various
  macros, OpenBSDInstallWrapup, OpenBSDInstallAperture, OpenBSDInstallConsole
  and OpenBSDInstallWrapupPart2.
- use the above macros when appropriate, OpenBSDInstallWrapup* being
  mandatory on all arches.
- remove serial console configuration blurb on sparc and sparc64, it is
  handled by OpenBSDInstallConsole now.