Minor touch-ups for VG_(record_startup_wd). 
- Use 500 instead of PATH_MAX as the initial size of the buffer 
  to hold the directory name. Should be more than enough in the
  majority of cases.
- Use VG_(realloc) instead of VG_(arena_realloc).


git-svn-id: svn://svn.valgrind.org/valgrind/branches/BUF_REMOVAL@14577 a5019735-40e9-0310-863c-91ae7b9d1cf9