config: fix TLSVersion.String() printing pointer address

[s3onghyun]

What

TLSVersion.String() falls back to fmt.Sprintf("%d", tv) for a version not present in the TLSVersions map — but tv is the *TLSVersion pointer, so it prints the pointer's memory address as a decimal integer instead of the numeric TLS version.

func (tv *TLSVersion) String() string {
	if tv == nil || *tv == 0 {
		return ""
	}
	for s, v := range TLSVersions {
		if *tv == v {
			return s
		}
	}
	return fmt.Sprintf("%d", tv) // <- prints &tv address, not *tv
}

The sibling methods MarshalYAML and MarshalJSON already dereference correctly; String() was the lone outlier, and the existing test only covered the known-value path so it was never caught (go vet doesn't flag it either).

Fix

Dereference the pointer: tv → *tv.

Test

Added TestTLSVersionStringerUnknown. Before the fix it printed a pointer address (e.g. "75679541043424"); after, it correctly returns "39321". go test ./config/ is green.