Add configurable connection timeout

[sollie]

Quick hack to add configurable connection timeout. Useful in scripts when scanning a ramge of hosts.

[TrueSkrillor]

As tscanner is supposed to be an importable library, I'd like to keep the interface stable. As Golang does not support optional parameters (or default values), I'd suggest that we introduce this feature as another function, ScanWithTimeout; Scan will then call ScanWithTimeout internally. Otherwise, I'm happy to merge these changes.

[sollie]

As Golang does not support optional parameters (or default values)

One could turn Scan() into a variadic function, and only care about the first argument.

Something like

func Scan(address string, scanMode ScanMode, verbose bool, timeout ...int) (*Report, error) {
	if len(timeout) == 1 {
		// set timeout
	} else if len(timeout) > 1 {
		// error out, more parameters than expected
	} else {
		// use default timeout
	}

For simplicity the default timeout could be specified in Scan().
I don't think this would have any downsides?

[TrueSkrillor]

While this would be possible, it seems overly complicated for what this is trying to achieve. I decided to add ScanWithTimeout as a separate function, converting Scan to a placeholder calling ScanWithTimeout instead. Also, I changed the parameter to be of type int, not *int to allow literal values to be passed to the function.