Fix domain name command line argument handling

[mattias-p]

Purpose

This PR fixes a couple of issues related to command line argument handling.

Context

This PR fixes items 1 and 2 in #364.

Changes

• Domain names "" and "0" are not rejected as missing domain names.
• The diagnostics/results separator is printed after input validation is completed.
• As a drive-by change, the unused Test::Reflow dependency is also removed.

How to test this PR

Zonemaster CLI should present the following behaviors:

$ zonemaster-cli --show-testcase --no-ipv6 --test basic/basic01 
Must give the name of a domain to test.

$ zonemaster-cli --show-testcase --no-ipv6 --test basic/basic01 ''
Domain name is empty


$ zonemaster-cli --show-testcase --no-ipv6 --test basic/basic01 0                                                                                                                                      

Seconds Level    Testcase       Message
======= ======== ============== =======
...

[marc-vanderwal]

Testing was successful. Each of the command lines in the testing procedures yielded the expected output.