Sync: Add some argument parsing to sync(1)

You can specify --block to make the program wait until sync is
complete.

userspace/sync/main.cpp

@@ -1,6 +1,29 @@
+#include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 
-int main()
+void usage(int ret, char* cmd)
 {
-	sync();
+	FILE* fout = (ret == 0) ? stdout : stderr;
+	fprintf(fout, "usage: %s [OPTION]...\n", cmd);
+	fprintf(fout, "Tells the kernel to start a disk sync as soon as possible\n");
+	fprintf(fout, "    -b, --block     return only after sync is complete\n");
+	fprintf(fout, "    -h, --help      show this message and exit\n");
+	exit(ret);
+}
+
+int main(int argc, char** argv)
+{
+	bool block = false;
+	for (int i = 1; i < argc; i++)
+	{
+		if (strcmp(argv[i], "-b") == 0 || strcmp(argv[i], "--block") == 0)
+			block = true;
+		else if (strcmp(argv[i], "-h") == 0 || strcmp(argv[i], "--help") == 0)
+			usage(0, argv[0]);
+		else
+			usage(1, argv[0]);
+	}
+	syncsync(block);
+	return 0;
 }