Set buffering on stdout and stderr to line-buffered.

Previously stdout would be block buffered and stderr unbuffered when writing to a non-terminal.
This code works on Debian with libc 2.11.2. The man page of setvbuf mentions that NULL can be used to avoid affecting the used buffer, but doesn't mention whether the size argument is then ignored; testing on Debian has revealed that this code works as intended, but I don't know whether that's standard.
(Christian Jaeger)

src/redshift.c

@@ -657,6 +657,13 @@ main(int argc, char *argv[])
 	int verbose = 0;
 	char *s;
 
+	/* Flush messages consistently even if redirected to a pipe or
+	   file.  Change the flush behaviour to line-buffered, without
+	   changing the actual buffers being used.
+	*/
+	setvbuf (stdout, NULL, _IOLBF, 0);
+	setvbuf (stderr, NULL, _IOLBF, 0);
+	
 	/* Parse command line arguments. */
 	int opt;
 	while ((opt = getopt(argc, argv, "b:c:g:hl:m:oO:rt:vVx")) != -1) {