Refactor the features configuration scheme.

Refactor the features configuration scheme.

Features configuration is handled in its own API in
features_config.h and features_config.c. This way, features
configuration is accessible to anything that needs it.

In addition, features configuration has been altered to
be more channel-oriented. Most callers of features API
code will be supplying a channel so that the individual
channel's settings will be acquired rather than the global
setting.

Missing from this commit is XML documentation for the
features configuration. That will be handled in a separate
commit.

Review: https://reviewboard.asterisk.org/r/2578/

(issue ASTERISK-21542)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390751 65c4cc65-6c06-0410-ace0-fbb531ad65f3