Validate flags after setting defaults

[phillip-stephens]

In zmap/zgrab2#632 we were seeing behavior where the ZCommander Validate function was showing that all the default values for flags weren't set.

It appears the issue is that we're calling Validate before setting default values on commands

Changes

• Set all default values before validating commands
• Only validate commands within eachCommand, eachOption is really inefficient if we care only about the command level
• Add a unit test that simulates the issue we saw in Default arguments don't work properly with multiply declared groups #3 so we can be sure this doesn't happen again