Fix adb shell escape handling.

Start of input is, by definition, the start of a line. Give the states
names to make the error clearer. We should also swallow the escape
character until/unless it's proven not to have been part of an escape
sequence. Finally, implement an ssh(1)-like -e option so the user can
choose -- or, more importantly, disable -- escape character handling.

Change-Id: I28a91db47e15f3e7de96303c0b9a98d302b9c92e