fix arity checking of `call-with-file-lock/timeout'
move "plt-services" and "distro-build" out of the distribution

The default `PKGS' value in the makefile now refers to both
"main-distribution" and  "plt-services", so the effect of a
plain `make' remains the same.
change farm-configuration file to a module

Also, improve connection between installers and farm-configuration
entries, and improve configuration of installer name versus
target-install directory.