feat: Make net_(tcp|udp) read limit configurable

[rexagod]

Defaults to 4GiB otherwise, as earlier.

Fixes: #364

[rexagod]

Rebased. Ready for reviews.

[discordianfish]

I think we should just implement the ptr function ourselves, it's just this:

// To returns a pointer to the given value.
func To[T any](v T) *T {
	return &v
}

[discordianfish]

LGTM

[SuperQ]

We have an internal/util package. Perhaps this should go there?

[SuperQ]

I'm not sure I like how this changes the public interface of the functions. Using a nil to specify "Use the default" is not great for library usability.

IMO, I don't think we should change this. Maybe better we should mark this function as not recommended or even deprecated (with no intention to remove).

In the node_exporter we stopped using these functions in favor of reading the data via netlink (github.com/mdlayher/netlink). This is a much more reliable method as well as it doesn't have the kernel locking issues that necessitate the reading method we use here.

See: prometheus/node_exporter#2322

[rexagod]

Initially, I did have doubts around changing the public signature, even though we don't guarantee any backward compatibilities, but I decided to go forward with the patch anyway since that was the only way of addressing the issue (there was the possibility of adding readLimit to FS, but that made little sense as it wasn't at all cohesive).

Seeing that we use netlink as a replacement for this, I'm fine with closing out this PR and adding a deprecation notice. Thank you for all the details.