conf: use typedefs for enums in "src/conf/nwfilter_conf.h"
    
    In "src/conf/" there are many enumeration (enum) declarations.
    Similar to the recent cleanup to "src/util" directory, it's better
    to use a typedef for variable types, function types and other
    usages. Other enumeration and folders will be changed to typedef's
    in the future. Most of the files changed in this commit are related
    to network filter (nwfilter_conf) enums.
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Signed-off-by: Eric Blake <eblake@redhat.com>