shred,sort,shuf: don't use /dev/urandom by default

shred,sort,shuf: don't use /dev/urandom by default

Suggestion from Steven Schveighoffer at:
http://savannah.gnu.org/patch/?6797
to greatly speed up the random passes done by shred.
* gl/lib/randread.c: Default to using the internal
pseudorandom generator, rather than reading /dev/urandom
* src/shred.c (usage): remove mention of /dev/urandom
* src/shuf.c (usage); ditto
* src/sort.c (usage): ditto
* doc/coreutils.text: Document the new behaviour
for aquiring random data.