[dash] Remove unused controller from dash

Change-Id: I20e9947aafd2f177e5632bfb7532a95077baa970

[dash] Add a shell controller protocol.

This patch adds a simple controller protocol, allowing to exchange
control messages with dash by passing a channel handle as
MX_HND_TYPE_USER1.

The protocol is described in `controller.h`, and currently only includes
commands sent from the shell to the controller:

- "get_history" retrieves the shell history.
response payload: empty
response handles: a vmo where shell history is stored as '\n'
separated entries, including a trailing '\n' after the last entry.
- "add_to_history:<entry>" adds the given <entry> to the shell
history.

Together, these allow the controller to manage the shell history (for
example, for persistance and synchronization).

Change-Id: I7c1d603d69e642a2140609c5718008d410c66d52

[dash] Add linenoise tab completion to dash

Change-Id: I6731af42100aed4a02080cbf8a6caa3c8b20afd7

[dash] Add builtin k function

The k function triggers a kerneldebug command, similar to ` in mxsh. We
can't use ` in dash because ` is a control character in Bourne shells.

Change-Id: I179ef72170b2a4b7a35e83f78181826397fa5f19

[dash] Add dash to the build

This patch integrates dash with the build system, including
pre-generating source files.

Change-Id: If19d0be0c030c6c194cb7bf66a26be48a39c0847