ship an xtrekrc system-wide

Customisation of the client using configuration files is possible, but without a system-wide xtrekrc file it is difficult for a user to discover how to begin customisation.

1. generate an xtrekrc file automatically during build, 2. place the file during install in /etc/netrek-client-cow/xtrekrc, 3. at startup, search for the file first in ~ then in /etc, 4. add customisation section to a manual page.