go: improve serial console

[djs55]

The current Go API for setting up the serial console has limitations, including:

• it supports at most one serial port
• it requires either stdio or a tty, and will trigger an assert if you don't provide one of them
• it combines the request for a tty with the request to log the output to a ring-buffer or to Apple System Log.

This PR adds a new Go API while keeping (but deprecating) the old one. It's now possible to have multiple serial ports, each of which is connected to one of:

• /dev/null
• a TTY
• stdio

and of which can be separately logged to any combination of:

• /dev/null
• a ring buffer
• Apple System Log

This PR also adds a go.mod and some unit tests for the console-related command-line building functions.

[djs55]

User observed a problem here: docker/roadmap#12 (comment) -- it looks like I've forgotten to modify the execute() function.
Unit tests are currently failing:

=== RUN   TestLegacyConsole
--- FAIL: TestLegacyConsole (0.00s)
    hyperkit_test.go:12: 
        	Error Trace:	hyperkit_test.go:12
        	Error:      	Expected nil, but got: &errors.errorString{s:"Could not find hyperkit executable hyperkit: exec: \"hyperkit\": executable file not found in $PATH"}
        	Test:       	TestLegacyConsole
=== RUN   TestNewSerial
--- FAIL: TestNewSerial (0.00s)
    hyperkit_test.go:21: 
        	Error Trace:	hyperkit_test.go:21
        	Error:      	Expected nil, but got: &errors.errorString{s:"Could not find hyperkit executable hyperkit: exec: \"hyperkit\": executable file not found in $PATH"}
        	Test:       	TestNewSerial
=== RUN   TestNullSerial
--- FAIL: TestNullSerial (0.00s)
    hyperkit_test.go:35: 
        	Error Trace:	hyperkit_test.go:35
        	Error:      	Expected nil, but got: &errors.errorString{s:"Could not find hyperkit executable hyperkit: exec: \"hyperkit\": executable file not found in $PATH"}
        	Test:       	TestNullSerial
FAIL
FAIL	github.com/moby/hyperkit/go	0.148s
FAIL
make: *** [test] Error 1

[mat007]

Looks good!

[mat007]

nit: } else if { works as well

[djs55]

Thanks!