update_server: Rewrote about 50% of the script, now works with RO checkouts

 * Using scons -Y to point scons to a source tree that might be
   read-only for the user building campaignd/wesnothd (as is the case on
   baldras).
 * Renamed or changed several variables for clarity.
 * As a side-effect, links within the server instance dir are now
   absolute rather than relative (/home/wesnoth/build/foo/bar instead of
   ../../build/foo/bar).
 * ~/builds-int/<version> is the intermediate build directory
 * ~/builds/<build-id> remains as the target directory
 * /usr/src/wesnoth is the source directory base (we need to handle
   basilic and gonzo separately later)

I'm fairly sure nobody other than me could possibly demand a more
detailed description of this commit.