uptime: refactor uumain

[cakebaker]

This PR refactors uumain. It applies the following changes:

• use clap to handle the "too many paths provided" error case
• remove unix block and make a single var platform-specific instead
• split default_uptime into two functions

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)