df: improve performance with many mount points

Use hash table for seaching in filter_mount_list() and get_dev()

This improves performance for 20K mount entries from:
  real    0m1.731s
  user    0m0.532s
  sys     0m1.188s
to:
  real    0m1.066s
  user    0m0.028s
  sys     0m1.032s

* src/df.c (devlist_table): Define hash table.
(devlist_hash): Add hash function.
(devlist_compare): Add hash comparison function.
(devlist_for_dev): Add lookup function.
(devlist_free): Add cleanup function.
(filter_mount_list): Use the above hash table.
While at it, rename the variable 'devlist' to 'seen_dev' for
better readability.
(me_for_dev): Use the above lookup function.
NEWS: Mention the improvement.
THANKS.in: Remove the committer; add original submitter Josef Cejka.

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