Window Install: Tidy up for Wix V3

Remove now optional attributes in <RegistryKey>

Change-Id: I1be890072e5b2cb8279d8fe02f473b095d5a180f
Windows Installer: Build a Command line merge module

Build a merge module which installs all of the Heimdal command
line tools.

Change-Id: I42297ce6fe32eed77bffb1a982d56d7e024789cc
roken: introduce rk_cloexec_socket

Accepts type rk_socket_t and is a no-op on Windows

Change-Id: I82755872e86ef36445e60054dd752279aaceb6d1