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/sh.cpp

@@ -147,7 +147,7 @@ int main(int, char**)
     int linedx = 0;
     linebuf[0] = '\0';
 
-    int fd = open("/dev/keyboard");
+    int fd = open("/dev/keyboard", O_RDONLY);
     if (fd == -1) {
         printf("failed to open /dev/keyboard :(\n");
         return 1;