maint: avoid "definitely lost" valgrind warnings

maint: avoid "definitely lost" valgrind warnings

Since commit v8.23-19-g8defcee, main() will return,
rather than call exit(), this inducing "definitely lost"
warnings in valgrind's leak checker.  That precludes using
the following to flag memory leaks:

  valgrind --leak-check=full --error-exitcode=1 \
           --errors-for-leak-kinds=definite

* src/pr.c (main): In dev builds, explicitly free memory allocated.
* src/sort.c (main): Likewise.
* src/tail.c (main): Likewise.
* src/tsort.c (tsort): Likewise.