lxc enter: improve escape key handling

lxc enter: improve escape key handling

The escape key should not be passed through, this now only
happens when pressing the escape key twice (as it is with
tmux, screen, ssh, etc.).
Also prepared the code to handle a configurable escape key,
and restructured the handling to more easily allow adding
more keys. For instance we could add shortcuts for some
`pct` commands. Most of them are marginally useful, but
snapshot, migrate, shutdown or stop might still be
nice-to-have things.
The idea is also that if we reuse the code for serial
terminals for VMs we can add shortcuts to eg. paste the
current terminal size (probably the most useful one), or
to send sys-rqs or agent commands.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>