examples: fix/silence -Wsign-conversion

[vszakats]

• extend FD_SET() hack to all platforms (was only Cygwin).
  Warnings may also happen in other envs, e.g. OmniOS.
  Ref: https://github.com/libssh2/libssh2/actions/runs/8854199687/job/24316762831#step:3:2021

• tidy-up CURLcode vs int use.

• cast an unsigned to long before passing to curl_easy_setopt().

Cherry-picked from #13489
Follow-up to 3829759 #12489
Closes #13501

[vszakats]

If no objections I'd go along merging this.

It has legitimate fixes and there is only one sign cast.

This one, if undesired, can be deleted or beefed up with sign checks. It's used
when converting the return value of if_nametoindex (int or long unsigned
depending on platform) to the long needed by curl_easy_setopt().