Remove --sudoers, improve --sudoers-no-modify

[skuhl]

Allowing sshuttle to add/overwrite sudoers configuration file at
locations of the users' choosing adds complexity to the code compared
to asking users to install the sudo configuration themselves. It
requires sshuttle to make decisions about how much effort we put into
ensuring that the file is written to a proper location. The current
method relies on the 'realpath' program which is not installed on
MacOS by default.

There are serious problems when the sudo configuration is used to
allow a user to only run sshuttle as root (with or without a
password). First, that user could then use the --sudoers option to
give other users sudo privileges. Second, the user can run any command
as root because sshuttle accepts a --ssh-cmd parameter which allows a
user to specify a program that sshuttle should run. There may also be
additional issues that we have not identified.

By removing the --sudoers option (and the associated sudoers-add
script), this reduces the problems above. This code keeps the
--sudoers-no-modify feature which prints a configuration to stdout for
the user to install. It includes a clear warning about how --ssh-cmd
could potentially be abused to run other programs.

A warning about some of these issues has been in sshuttle since
version 1.1.0. This commit also adds that warning to more locations in
the documentation.

[brianmay]

I haven't seen any objections to this proposal in any of the discussions anywhere. Will wait until tomorrow morning, then merge.