du: tune, and fix some -L bugs with dangling or cyclic symlinks

* src/du.c (process_file): Avoid recalculation of hashes
and of file-exclusion for directories.  Do not descend into
the same directory more than once, unless -l is given; this is faster.
Calculate stat buffer lazily, since it
need not be computed at all for excluded files.
Count space if FTS_ERR, since stat buffer is always valid then.
No need for 'print' local variable.
(main): Use FTS_NOSTAT.  Use FTS_TIGHT_CYCLE_CHECK only when not
hashing everything, since process_file finds cycles on its own
when hashing everything.
* tests/du/deref: Add test cases for -L bugs.

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