Function one_input_line

** Retrieve a single line of input text. ** ** If in==0 then read from standard input and prompt before each line. ** If isContinuation is true, then a continuation prompt is appropriate. ** If isContinuation is zero, then the main prompt should be used. ** ** If zPrior is not NULL then it is a buffer from a prior call to this ** routine that can be reused. ** ** The result is stored in space obta