Simple binding for sendfile() of Linux. Used system calls:
EntireFile -- open(), stat(), sendfile(), and close()
PartOfFile -- open(), sendfile(), and close()
If the size of the file is unknown when sending the entire file, specifying PartOfFile is much faster.
The fourth action argument is called when a file is sent as chunks. Chucking is inevitable if the socket is non-blocking (this is the default) and the file is large. The action is called after a chunk is sent and before waiting the socket to be ready for writing.