Change VG_(mkstemp) such that
(a) the 2nd argument must not be NULL
    This was true anyhow and requiring it allows us to simplify the function
    by eliminating the local buffer.
(b) the memory pointed to by the 2nd argument is always initialised
    In the past the output file name was not initialised in case VG_(open)
    failed 10 times in a row. The call sites in m_main.c and m_gdbserver/target.c
    were reading the uninitialised filename unconditionally. This was spotted
    by IBM's BEAM checker.
Fix call sites, eliminate some magic constants along the way.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14706 a5019735-40e9-0310-863c-91ae7b9d1cf9