Expose the kernel log buffer through /proc/dmesg.

Also add a /bin/dmesg program for convenience.

Userland/dmesg.cpp

@@ -0,0 +1,31 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <assert.h>
+
+int main(int argc, char** argv)
+{
+    (void) argc;
+    (void) argv;
+    int fd = open("/proc/dmesg", O_RDONLY);
+    if (fd < 0) {
+        perror("open /proc/dmesg");
+        return 1;
+    }
+    for (;;) {
+        char buffer[BUFSIZ];
+        ssize_t nread = read(fd, buffer, sizeof(buffer));
+        if (nread < 0) {
+            perror("read");
+            return 1;
+        }
+        if (nread == 0) {
+            break;
+        }
+        ssize_t nwritten = write(1, buffer, nread);
+        assert(nwritten == nread);
+    }
+    int rc = close(fd);
+    assert(rc == 0);
+    return 0;
+}