Add basic symlink support.

- sys$readlink + readlink()
- Add a /proc/PID/exe symlink to the process's executable.
- Print symlink contents in ls output.
- Some work on plumbing options into VFS::open().

Userland/cat.cpp

@@ -9,7 +9,7 @@ int main(int argc, char** argv)
         printf("usage: cat <file>\n");
         return 1;
     }
-    int fd = open(argv[1]);
+    int fd = open(argv[1], O_RDONLY);
     if (fd == -1) {
         printf("failed to open %s: %s\n", argv[1], strerror(errno));
         return 1;