This commit fixes cmCTestRunTest and cmProcess to more efficiently
handle child output. We now use the buffer for each child output pipe
to hold at most a partial line plus one new block of data at a time.
All complete lines are scanned in-place, and then only the partial line
at the end of the buffer is moved back to the beginning before appending
new data.
We also simplify the cmProcess interface by making GetNextOutputLine the
only method that needs to be called while the process is running. This
simplifies cmCTestRunTest so that CheckOutput can be called until it
returns false when the process is done.
Brad King
Bill Hoffman
Zach Mullen