Problem: The "last used" info of a buffer is under used.
Solution: Add "lastused" to getbufinfo(). List buffers sorted by last-used
field. (Andi Massimino, closes#4722)
Problem: Some source files are too big.
Solution: Move buffer and window related functions to evalbuffer.c and
evalwindow.c. (Yegappan Lakshmanan, closes#4898)