Log Message:
lang/racket, lang/racket-minimal: Update to 8.15

While here:

1. Patch around https://github.com/racket/racket/issues/5122 (seek in
   boot file petite failed) and
   https://github.com/racket/racket/issues/5049 (8.13 fails to build
   on NetBSD) by using argv[0] instead of
   realpath("/proc/curproc/file") as the reference point for
   executable-relative paths.

1. Work around https://github.com/racket/racket/issues/4290
   (Regression since Racket 8.5: fonts are required to build
   successfully) by pulling in a small font at build-time.

3. Add PRINT_PLIST_AWK to assist with future updates.