Low-level implementation of Stdio.sendfile() .
Sends headers followed by len bytes starting at offset
from the file from followed by trailers to the file to .
When completed callback will be called with the total number of
bytes sent as the first argument, followed by args .
Any of headers , from and trailers may be left out
by setting them to 0
.
Setting offset to -1
means send from the current position in
from .
Setting len to -1
means send until from 's end of file is
reached.