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

conf: use typedefs for enums in "src/conf/snapshot_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 snapshot (snapshot_conf) enums.

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>