add hook statement to configuration parser

add hook statement to configuration parser

- allow multiple "hook" statements at global or view level
- add "optional bracketed text" type for optional parameter list
- load hook module from specified path rather than hardcoded path
- add a hooktable pointer (and a callback for freeing it) to the
  view structure
- change the hooktable functions so they no longer update ns__hook_table
  by default, and modify PROCESS_HOOK so it uses the view hooktable, if
  set, rather than ns__hook_table. (ns__hook_table is retained for
  use by unit tests.)
- update the filter-aaaa system test to load filter-aaaa.so
- add a prereq script to check for dlopen support before running
  the filter-aaaa system test

not yet done:
- configuration parameters are not being passed to the filter-aaaa
  module; the filter-aaaa ACL and filter-aaaa-on-{v4,v6} settings are
  still stored in dns_view