Avoid fixed size buffer startup_wd for VG_(record_startup_wd).
Change VG_(get_startup_wd) to simply return the string.
Fix call sites.

We could even fold both functions into a single one with the
benefit of giving the two associated global variables function
scope.


git-svn-id: svn://svn.valgrind.org/valgrind/branches/BUF_REMOVAL@14206 a5019735-40e9-0310-863c-91ae7b9d1cf9