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 Changed parameters of f_run. Changed rtein+rteout pair to rte pointer
passed by reference, added ea_list of temporary attrs again passed
by reference and finally added a pointer to memory pool for storing
temporary data (new ea_lists's, temporary rta's etc.).
o Re-ordered result codes, so that all accepts come before all rejects.
o Introduced FILTER_ACCEPT and FILTER_REJECT dummy values (will be
used in protocol configurations).
o Added filter_name() which returns name of a filter or ACCEPT/REJECT
for the dummies.
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.
