go.crypto/ssh/terminal: Allow ^A and ^E as synonyms for Home and End.

I understand that ssh/terminal can't implement everybodys
favorite keyboard shortcuts, but I think these are very
widespread. They exist not only in Emacs or Readline, but also
in Acme and Sam. Also they almost come for free.

R=golang-dev
CC=agl, golang-dev
https://golang.org/cl/13839047