Package-database reading and related library improvements

The main result is that `path->selative-string/library' now generates
"<pkg>/..." strings.

To make that possible, minimal support for reading the
installed-package database moved to a new `pkg/path' library.

To make that possible and avoid poetntial problems from concurrent
reading and writing of the database file, `racket/file' now provides
`call-with-atomic-output-file', which implements the write-and-move
dance that CM uses for compiled bytcode (i.e., `with-compile-output').
The new function is used to write links and installed-package files.

Finally, `racket/base' provides the new function
`call-with-default-reading-parameterization', which is used to guard
various file `read's to make them consistent and avoid security holes.