Fix some issues related to dynamic feature groups in features.conf.

The bridge handling code did not properly consider feature groups when setting
parameters that would affect whether or not a native bridge would be attempted.
If DYNAMIC_FEATURES only include a feature group, a native bridge would occur
that may prevent features from working.

Fix a bug in verbose output that would show the key mapping as empty if it was
using the default mapping and not a custom mapping in the feature group.

Add feature groups to the output of "features show".

Adjust the feature execution logic to match that of the logic when executing
a feature that was not configured through a feature group.

Update features.conf.sample to show that an '=' is still required if using
the default key mapping from [applicationmap].

Finally, clean up a little bit of formatting to better coform to coding
guidelines while in the area.

(closes issue #17589)
Reported by: lmadsen
Patches:
      issue_17589.rev4.txt uploaded by russell (license 2)
Tested by: russell, lmadsen


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@275424 65c4cc65-6c06-0410-ace0-fbb531ad65f3