Kernel: Pass correct permission flags when opening files

Right now, permission flags passed to VFS::open() are effectively ignored, but
that is going to change.

* O_RDONLY is 0, but it's still nicer to pass it explicitly
* POSIX says that binding a Unix socket to a symlink shall fail with EADDRINUSE

Tests/Kernel/bind-local-socket-to-symlink.cpp

@@ -0,0 +1,31 @@
+#include <stdio.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+int main(int, char**)
+{
+    constexpr const char* path = "/tmp/foo";
+    int rc = symlink("bar", path);
+    if (rc < 0) {
+        perror("symlink");
+        return 1;
+    }
+
+    int fd = socket(AF_UNIX, SOCK_STREAM, 0);
+    if (fd < 0) {
+        perror("socket");
+        return 1;
+    }
+
+    struct sockaddr_un addr;
+    memset(&addr, 0, sizeof(addr));
+    addr.sun_family = AF_UNIX;
+    strncpy(addr.sun_path, path, sizeof(addr.sun_path) - 1);
+
+    rc = bind(fd, (struct sockaddr*)(&addr), sizeof(addr));
+    if (rc < 0 && errno == EADDRINUSE)
+        return 0;
+
+    return 1;
+}