VG_(memset) cannot handle NULL pointers and needs to be guarded.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13556 a5019735-40e9-0310-863c-91ae7b9d1cf9
Followup to r13553 which caused some build failures.
(1) Detect availability of pthread_setname_np. Ignore testcases
    memcheck/tests/threadname[_xml] if not available.
(2) Enable _GNU_SOURCE to avold compiler warnings.
(3) In threadname_xml filter out stackframes referring to system
    libraries. Added tests/filter_xml_frames to do that.
(4) Adjust .exp files as needed
(5) Do not ship stdout.exp for memcheck/tests/threadname[_xml].


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13557 a5019735-40e9-0310-863c-91ae7b9d1cf9