filter-expr-grammar: extract filter_fac rule

This patch extracts the bit-banging parts from the filter_fac_list rule
into a separate, new rule named filter_fac. This removes some code
duplication and prepares the scene for introducing facility range support.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
filter-expr-grammar: add support ranges in the facility() filter

This way one can easily grab facility(local0..local6). It can be used
for all the other facility codes, though it might make less sense,
as facilities are ordered by their numerical codes.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>