mysqlimport had code for multi-threaded import. By mistake it was disabled for
many years, at least since 5.5 (more likely even in 5.1), but mysqlimport happily
accepted (and ignored) --use-threads option ever since.

After Mroonga merge HAVE_LIBPTHREAD became defined and multi-threaded import
suddenly came to life. As it exit() the program brutally on any error (never mind
that some import threads may be running) mysys rightfully complains. Safemalloc
complains too in debug builds.

Fix: don't try a clean exit on an error, don't shutdown mysys and tell safemalloc
to shut up. Yes, and remove #ifdef HAVE_LIBPTHREAD, since 5.5 the client library
is always multi-threaded.