xz: Don't fail if stdin doesn't support O_NONBLOCK.

It's a problem at least on OpenBSD which doesn't support
O_NONBLOCK on e.g. /dev/null. I'm not surprised if it's
a problem on other OSes too since this behavior is allowed
in POSIX-1.2008.

The code relying on this behavior was committed in June 2013
and included in 5.1.3alpha released on 2013-10-26. Clearly
the development releases only get limited testing.

src/xz/file_io.c

diff --git a/src/xz/file_io.c b/src/xz/file_io.c
index f135cf7..bf4462d 100644
--- a/src/xz/file_io.c
+++ b/src/xz/file_io.c
@@ -393,7 +393,10 @@
 #ifdef TUKLIB_DOSLIKE
 		setmode(STDIN_FILENO, O_BINARY);
 #else
-		// Enable O_NONBLOCK for stdin.
+		// Try to set stdout to non-blocking mode. It won't work
+		// e.g. on OpenBSD if stdout is e.g. /dev/null. In such
+		// case we proceed as if stdout were non-blocking anyway
+		// (in case of /dev/null it will be in practice).
 		stdin_flags = fcntl(STDIN_FILENO, F_GETFL);
 		if (stdin_flags == -1) {
 			message_error(_("Error getting the file status flags "
@@ -402,17 +405,10 @@
 			return true;
 		}
 
-		if ((stdin_flags & O_NONBLOCK) == 0) {
-			if (fcntl(STDIN_FILENO, F_SETFL,
-					stdin_flags | O_NONBLOCK) == -1) {
-				message_error(_("Error setting O_NONBLOCK "
-						"on standard input: %s"),
-						strerror(errno));
-				return true;
-			}
-
+		if ((stdin_flags & O_NONBLOCK) == 0
+				&& fcntl(STDIN_FILENO, F_SETFL,
+					stdin_flags | O_NONBLOCK) != -1)
 			restore_stdin_flags = true;
-		}
 #endif
 #ifdef HAVE_POSIX_FADVISE
 		// It will fail if stdin is a pipe and that's fine.