a large change to make rsync much more memory efficient. This is done

in 3 ways:

1) the realloc is done on a list of pointers, not a list of structures
2) only the basename of the file is now kept in the file struct an a
   util function f_name() is now used to access the full name when
   required.
3) pointers to directory names are re-used

hopefully I haven't broken anything. This will need lots of testing.