conf: use typedefs for enums in "src/conf/nwfilter_conf.h"

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>