log: flush log lines, to cope with non-terminal output (pipes)

log: flush log lines, to cope with non-terminal output (pipes)

Although log messages are terminated by a line feed, output still might
get buffered when the program does not talk to a terminal (that it is
aware of). Add an fflush(3) call to the printf(3) sequence such that log
output will immediately show up when writing to pipes or sockets, too.