[FR] Use cobra cli framework for fatd cli flags

[AdamSLevy]

Is your feature request related to a problem? Please describe.
Currently fat-cli uses the cobra cli framework and uses unix style flags, but fatd uses just the plain "flags" package and some custom parsing. This means that there is some code duplication for common flags (which may be unavoidable in some places anyway) but mainly this creates an unfortunate user experience when switching between the two.

Describe the solution you'd like
Re-implement the fatd cli parsing to use the cobra cli framework similar to fat-cli.