more tests for libdb btree page split on zero index

more tests for libdb btree page split on zero index

Enhance btree debugging output somewhat to limit key printout to the
key length if the key is not null-terminated.

Add additional test case for the zero-index page split bug; test case
can create a corrupted btree database with records unreachable by
random access but reachable by sequential access.  Requires
recompiling with CPPFLAGS='-DDEBUG -DDEBUG_IDX0SPLIT' to correctly
model mpool page reuse that would be present in production conditions.
(CPPFLAGS=-DDEBUG would otherwise explicitly overwrite the contents of
reused pages.)

ticket: new
target_version: 1.6.4
tags: pullup
component: krb5-kdc

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20222 dc483132-0cff-0310-8789-dd5450dbe970