when we get eof, stop the loop immidiately and do not
rely on the read to eventually return an error.
when convM2S() fails to decode the message, error out
and stop the loop. there is no point in continuing.
maximum file size is 4GB-1 as the file length is stored in
a 32 bit long. make sure it doesnt overflow on write or
or truncate. interpret the file length as unsigned. pass
vlong to readfile()/writefile()/truncfile() so we can
handle overflows and not just ignore the upper bits.
