o cf_define_symbol() -- it assigns a meaning to a symbol, bailing
out if it already has one.
o cf_find_symbol() -- finds symbol by name and creates it if not found.
Also modified filter/config.Y to make use of the first function.
o Introduced struct filter which serves as an external reference
to filter. Using struct symbol for this is unwise since it doesn't
allow extra information attached to the filter and it also forces
all filters to be named.
o Implemented config rule 'filter' which matches either named filter
or an embedded unnamed filter (`{ <filter> }').
o Fixed totally bogus comment at the top of filter.h.
o Added a missing prototype for f_run() to filter.h.
