curl: make --silent work stand-alone

curl: make --silent work stand-alone

- renamed the struct field to 'silent' to match the cmdline option
- make --show-error toggle independently of --silent
- make --silent independent of ->noprogress as well

By doing this, the three options --silent, --no-progress-meter and
--show-error should work independently of each other and also work with
and without '--no-' prefix as documented.

Reported-by: u20221022 on github
Fixes #10535
Closes #10536