Make the location for temporary files configurable at runtime.

Make the location for temporary files configurable at runtime.
To that effect observe the environment variable TMPDIR. If defined,
its value takes precedence over VG_TMPDIR.
Because the directory name is no longer a compile time constant,
VG_(err_config_error) was changed to take a variable argument list.
Fixes #267020.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11892