api: Add api convenience call for renaming channels. (#4715).

api: Add api convenience call for renaming channels. (#4715).

Add a convenience function for users to rename all channels
that match a particular name. Previously the caller needed
to parse an api/channel/list, extract each channel that
matched the name and then issue an idnode/save for the nodes.

This change allows people to easily fixup their names such as
updating channels that are broadcast with a lowercase name
(and so sort to the end of the list in many UIs); or prefixing
the channels with identifiers. Since the server does the channel
name lookup, it allows non-programmers to issue simple renames
from a script.

curl http://l:9981/api/channel/rename --data-urlencode 'from=dave' --data-urlencode 'to=Dave'

Issue: #4715.