termproxy: switch from clap to pico-args for CLI argument handling

termproxy: switch from clap to pico-args for CLI argument handling

Not that clap is bad or anything the like, but for one it's rather
over engineered, and it has to be as long as it wants to provide a
dozen wildly different way to do things.
And the second, more important reason: it's still undergoing a lot of
churn every year or so.  Each upgrade to a major version needs like
two hours of understanding what's going on, at least if one wants to
Do It Right™.

Termproxy, otoh., is a small and internal tool that doesn't need an
overly fancy CLI targetting humans, as it will be only called by the
API anyway.

So, to reduce the time required to constantly catch up, and remove
some complexity, switch over to pico-args. That one provides a few
small interfaces for the most common things, does it right and uses
OsString as main type and has exactly zero dependencies on its own.
In other words, perfect for such internal tools (and possibly also
most others).

Copy over the help output from the clap based tool for convenience,
pico-args really doesn't bother with such things, and introduce an
Options struct to have a, well, more structured way of handling CLI
arguments/options.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>