Add some new editline bindings by default, and allow for user specified configuration.

I excluded the part of this patch that used the HOME environment variable since
the built-in editline library goes to great lengths to disallow that.  Instead
only settings the EDITRC environment variable will use a user specified file.

Also, the default environment variable use to determine the edit more is
AST_EDITMODE instead of AST_EDITOR (although the latter is still supported).

(closes issue #15929)
Reported by: kkm
Patches:
      astcli-editrc-v2.diff uploaded by kkm (license 888)
      015929-astcli-editrc-trunk.240324.diff uploaded by kkm (license 888)
Tested by: seanbright


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@317395 65c4cc65-6c06-0410-ace0-fbb531ad65f3