[omdb] improve CLI UX

[sunshowers]

• Use global = true for global options wherever possible -- this means that
  you can now pass in arguments like --nexus-db-url at the end.
• Add a few help headings to group together related options.

Depends on #5615 and #5620 (mostly incidentally).

[sunshowers]

Per discussion with @davepacheco, here's what will be accepted afterwards:

command	accept -w?	see -w in help?	accept --db-url?	see --db-url in help?
omdb	yes, anywhere	yes	no	no
omdb db	yes, anywhere	yes	yes, after db only	yes
omdb db sleds	yes, anywhere	yes	yes, after db or after sleds	yes
omdb nexus	yes, anywhere	yes	no	no
omdb nexus sleds	yes, anywhere	yes	no	no

[andrewjstone]

This is awesome. Thank you so much @sunshowers!

[sunshowers]

accidentally landed this onto the wrong branch, re-did in #5755