distro-build/farm: fix `#:pkgs' handling
distro-build/farm: improve break handling
raco pkg: fix handling of overlapping provided-package sets
change exports from the 'pkg' module so they are all prefixed with pkg-

(the goal is to avoid shadowing 'remove')
add transparent "color" to 2htdp/image.
Make struct: work at the REPL

Closes PR 11669
add a section to the image guide about rotation and centers
make doc-search URL configurable for installer builds

Also, add an initial-catalogs configuration to clients and
`farm' builds.
Generalize use of farm config file

Change `FARM_CONFIG' to just `CONFIG' and use it on the server, too.
racket/sandbox: repair for link search paths
allow any real number (except +inf.0, -inf.0, and +nan.0)
as angles

also bring image.scrbl down below 102 columns
clarify wording
Add travis config file.
Try more build options.