Refactorings to the filter code, most notably:

Refactorings to the filter code, most notably:

- Improve function names: parse_filter_file -> load_filter_file and
  parse_rule -> load_filter_str (to make the similarity clearer, since
  it /can/ load multiple rules when MATCHFLG_WORD_SPLIT is specified).

- In preparation for rule prefixes containing information beyond the
  mflags, change the code to pass around a full "template" filter_struct
  instead of just mflags.  Callers of load_filter_{str,file} that want
  to specify only mflags can use filter_template(mflags) .

- Remove the MODIFIERS_* strings and instead hand-code the condition
  under which each modifier is valid.  This should make it easier to
  see that the conditions are correct.

- Tighten up default modifiers on merge rules:
  - Disallow "!" because it isn't useful.
  - If the merge rule specifies a side via "s" or "r", the rules in the
    file cannot also specify a side via "s", "r", "hide", etc.