implement ast_waitfordigit() and ast_readstring() in terms

of their '_full()' version, so we can remove the replicated
implementation and, especially, the risk that they get out of sync.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@20389 65c4cc65-6c06-0410-ace0-fbb531ad65f3