- Read exactly one line and strip the trailing newline if any. If the
- file was on EOF, return NULL. Otherwise, return all the data in a
- dynamically allocated buffer.
-
- If line is non-NULL, it will be used as an initial buffer, to avoid
- unnecessary mallocing each time this function is called. If buf is
- given, and buf needs to be expanded, the var pointed to by buflen
- will be increased.