get-pkg-context: add `#:use-cache?` and `#:quiet?` arguments
doc clarification on `beg0` bytecode form

From Gustavo, and intended to be part of commit
2d95c39051.
pkg info.rkt: refine specification of `distribution-preference`
raco pkg: add trash directory for removed/updated package installations

Packages that are installed as other than a link are not meant to be
edited, but work can get lost if a package is edited and then removed
or updated. Avoid that work loss by moving removed or updated packages
to a trash folder.

By default, the trash folder holds up to 512 packages for up to 48
hours. To disable the trash folder (for a given scope), use

 raco pkg config --set max-trash-packages 0

(I expect that some variant of Greenspun's rule predicted the eventual
inclusion of "backup" management in the package system.)
fix thread-based place implementation

There are some limitations to the places implementation when
`(place-enabled?)` returns #f, but mostly it can work.