Add flags to selectp to enable and disable input to a pane, from Anish
Athalye.
Fix two copy mode problems:

1. In vi mode the selection doesn't include the last character if you
   moved the cursor up or left.
2. In emacs mode the selection includes the last character if you moved
   the cursor to the left.

From Balazs Kezes.
No need to repeat other-end more than once, from Juho Pohjala. Also add
it to the commands list while here.