The `QuotedString` option was added in [moby@e4c1f07] and [moby@abe32de]
to work around a regression in Docker 1.13 that caused `docker-machine`
to fail. `docker-machine` produced instructions on how to set up a cli
to connect to the Machine it produced. These instructions used quotes
around the paths for TLS certificates, but with an `=` for the flag's
values instead of a space; due to this the shell would not handle
stripping quotes, so the CLI would now get the value including quotes.

Preserving quotes in such cases is expected (and standard behavior), but
versions of Docker before 1.13 used a custom "mflag" package for flag
parsing, and that package contained custom handling for quotes (added
in [moby@0e9c40e]).

For other flags, this problem could be solved by the user, but as these
instructions were produced by `docker-machine`'s `config` command, an
exception was made for the `--tls-xxx` flags. From [moby-29761]:

> The flag trimming behaviour is really unusual, and I would say unexpected.
> I think removing it is generally the right idea. Since we have one very
> common case where it's necessary for backwards compatibility we need to
> add a special case, but I don't think we should apply that case to every
> flag.

The `QuotedString` implementation has various limitations, as it doesn't
follow the same handling of quotes as a shell would.

Given that Docker Machine reached EOL a long time ago and other options,
such as `docker context`, have been added to configure the CLI to connect
to a specific host (with corresponding TLS configuration), we should remove
the special handling for these flags, as it's inconsitent with all other
flags, and not worth maintaining for a tool that no longer exists.

This patch deprecates the `QuotedString` option and removes its use. A
temporary, non-exported copy is added, but will be removed in the next
release.

[moby-29761]: moby/moby#29761 (comment)
[moby@e4c1f07]: moby/moby@e4c1f07
[moby@abe32de]: moby/moby@abe32de
[moby@0e9c40e]: moby/moby@0e9c40e
[moby@c79a169]: moby/moby@c79a169

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>