i9e: Fix compilation on Ubuntu-12.04.

i9e: Fix compilation on Ubuntu-12.04.

Commit c0162946 (i9e: Avoid key binding macros) from half a year
ago broke compilation for readline-6.2, which ships at least with
Ubuntu-12.04. The problem is that c0162946 changed dispatch_key()
to use rl_executing_keyseq, a readline variable that was introduced
in readline-6.3. Compilation fails on systems with readline-6.2 or
older because the variable does not exist.

This patch modifies interactive.c to provide an equivalent of
rl_executing_keyseq and changes dispatch_key() to use this version
instead.