Port passthrough

[WhatAmISupposedToPutHere]

Add an ability to configure the set of published ports via command line switches.

This is a continuation from #134 that moves all internal communications to vsock ports.

[teohhanhui]

Does this subsume #134?

[WhatAmISupposedToPutHere]

Not really. Those are prs for different features, that just happen to be dependent on each other.

[teohhanhui]

I'm guessing this is a copy and paste error, right?

[WhatAmISupposedToPutHere]

yeah... unlikely to ever be hit, but should probably fix that one

[teohhanhui]

The POSIX spec says:

The value of an environment variable is an arbitrary sequence of bytes, except for the null byte.

https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html

So I think we can just change this to expect everywhere, e.g.

Suggested change

	.context("Failed to process `muvm-guest` path as it contains NUL characters")?;
	.expect("server_path should not contain NUL character");

(I can do this after the rest is merged, to not create more work for you...)

[teohhanhui]

As done elsewhere...

Suggested change

	.map_err(|e| anyhow!("unable to get XDG_RUNTIME_DIR: {:?}", e))?;
	.context("Failed to read XDG_RUNTIME_DIR environment variable")?;

This would already wrap the internal error.

(I can do this after the rest is merged, to not create more work for you...)

[teohhanhui]

Note to self:

Suggested change

	let optslash = if self.ip.is_empty() { "" } else { "/" };
	[
	if self.udp { "-u" } else { "-t" }.to_owned(),
	format!(
	"{}{}{}-{}:{}-{}",
	self.ip,
	optslash,
	self.host_range.0,
	self.host_range.1,
	self.guest_range.0,
	self.guest_range.1
	),
	]
	[
	if self.udp { "-u" } else { "-t" }.to_owned(),
	format!(
	"{ip}{slash}{host_start}-{host_end}:{guest_start}-{guest_end}",
	ip = self.ip,
	slash = if self.ip.is_empty() { "" } else { "/" },
	host_start = self.host_range.0,
	host_end = self.host_range.1,
	guest_start = self.guest_range.0,
	guest_end = self.guest_range.1
	),
	]