confbridge: Rename items for clarity and consistency.

confbridge: Rename items for clarity and consistency.

struct conference_bridge_user -> struct confbridge_user
struct conference_bridge -> struct confbridge_conference
struct conference_state -> struct confbridge_state

struct conference_bridge_user *conference_bridge_user -> struct confbridge_user *user
struct conference_bridge_user *cbu -> struct confbridge_user *user
struct conference_bridge *conference_bridge -> struct confbridge_conference *conference

The names are now generally shorter, consistently used, and don't conflict
with the struct names.

This patch handles the renaming part of the issue.

(issue ASTERISK-20776)
Reported by: rmudgett

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@382764 65c4cc65-6c06-0410-ace0-fbb531ad65f3