fix null character in object name triggering segfault

Parsing \n in  lfn_parse_object_name is implemented with

  out->append('\0');

which segfaults when using libstdc++ and g++ version 4.6.3 on Debian
GNU/Linux. It is replaced with

  (*out) += '\0';

to avoid the bugous implicit conversion. There is no append(charT)
method in C++98 or C++11, which means it relies on an implicit
conversion that is bugous. It would be better to rely on the
basic_string& operator+=(charT c); method as defined in ISO 14882-1998
(page 385) thru ISO 14882-2012 (page 640)

A set of tests is added to generate and parse object names. They need
access to the private function lfn_parse_object_name because there is
no convenient protected method to exercise it. The tests contain a
LFNIndex derived class, TestWrapLFNIndex which is made a friend of
LFNIndex to gain access to the private methods.

http://tracker.ceph.com/issues/4594 refs #4594

Signed-off-by: Loic Dachary <loic@dachary.org>