fix 320211 Stack buffer overflow in ./coregrind/m_main.c with huge TMPDIR
* Addition of a function to compute size of buffer needed for VG_(mkstemp)
* Use it to dimension buffers for all VG_(mkstemp) calls.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13409 a5019735-40e9-0310-863c-91ae7b9d1cf9