Get the second stage loader from the Load Options

This commit replaces the 2nd stage loader path with the first
argument in the Load Options and moves the rest arguments (if any)
to the Load Options for the 2nd stage loader.

For example, to make shim to load elilo.efi, just create a new
boot entry with efibootmgr:

 # efibootmgr -c -L "shim elilo" -l "efi\\shim.efi" -u "elilo.efi"
Clear the screen before erasing keys
Reboot the system after enrolling/erasing keys
Add a general function for password matching
MOK doesn't include the signature owner
Add support for deleting specific keys
Make sure the menu shows when the callback fails

Since Pause() doesn't clear the key from the input queue, the next
ReadKeyStroke reads the queued key instead of the new one. If the
user presses "Enter", MokManager exits directly without showing
the menu again.