syscall: Accept O_NONBLOCK flag in pipe2

pipe2 only accepted O_CLOEXEC, rejecting O_NONBLOCK with ENOTSUP.
Accept O_NONBLOCK and set it on the created pipe streams.

This is needed by GLib's GWakeup which uses
pipe2(fds, O_CLOEXEC | O_NONBLOCK).

Author: thiblahute

src/lib/libsyscall.js

@@ -199,12 +199,18 @@ var SyscallsLibrary = {
     if (fdPtr == 0) {
       throw new FS.ErrnoError({{{ cDefs.EFAULT }}});
     }
-    if (flags && flags != {{{ cDefs.O_CLOEXEC }}}) {
+    var validFlags = {{{ cDefs.O_CLOEXEC }}} | {{{ cDefs.O_NONBLOCK }}};
+    if (flags & ~validFlags) {
       throw new FS.ErrnoError({{{ cDefs.ENOTSUP }}});
     }
 
     var res = PIPEFS.createPipe();
 
+    if (flags & {{{ cDefs.O_NONBLOCK }}}) {
+      FS.getStream(res.readable_fd).flags |= {{{ cDefs.O_NONBLOCK }}};
+      FS.getStream(res.writable_fd).flags |= {{{ cDefs.O_NONBLOCK }}};
+    }
+
     {{{ makeSetValue('fdPtr', 0, 'res.readable_fd', 'i32') }}};
     {{{ makeSetValue('fdPtr', 4, 'res.writable_fd', 'i32') }}};