filter: reduce memory usage, fix OOM crashes

* check return values of memory allocation and abort gracefully
  in out-of-memory situations

* use sblist (linear dynamic array) instead of linked list
  - this removes one pointer per filter rule
  - removes need to manually allocate/free every single list item
    (instead block allocation is used)
  - simplifies code

* remove storage of (unused) input rule
  - removes one char* pointer per filter rule
  - removes storage of the raw bytes of each filter rule

* add line number to display on out-of-memory/invalid regex situation

* replace duplicate filter_domain()/filter_host() code with a single
  function filter_run()
  - reduces code size and management effort

with these improvements, >1 million regex rules can be loaded with
4 GB of RAM, whereas previously it crashed with about 950K.

the list for testing was assembled from
http://www.shallalist.de/Downloads/shallalist.tar.gz

closes #20

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