bring in the code that was discussed on Mantis #6068,

bring in the code that was discussed on Mantis #6068,
which is the basis for several simplifications and fixes
to the CLI interfaces.

The core is in cli.c, some documentation on a new function
to help command completion is in cli.h, and one line of
glue code in the other two files.

Next step is to bring in the patches described in #6066 and
other simplifications.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@15817 65c4cc65-6c06-0410-ace0-fbb531ad65f3