Add "exec" service: shell commands with no pty.

To facilitate device scripts that want to read/write binary data from
the host side, this change introduces a new "exec" service that
behaves like "shell" but without creating a pty, which would otherwise
mangle binary data.

After forking, it hooks up stdin/stdout of the child process to
the socket connected through to the host.  The adb transport doesn't
support shutdown(), so the host can't half-close the socket and wait
for device termination.  Instead, the host side now has two explicit
commands "exec-in" and "exec-out" for either sending or receiving
data.

Teach host side copy_to_file() to deal with stdin/stdout special
cases.  Switch device side backup/restore services to use the new
create_subproc_raw under the hood.

(cherry picked from commit 5d9d434e)

Change-Id: I42c18cb5bb907449b458c94450ef6c584d84ecdb