OS400 check dupstring enumeration at build time, not runtime

[jonrumsey]

"*** WARNING: curl_easy_setopt_ccsid() should be reworked ***" appears at runtime on OS400 when using curl_easy_setopt_ccsid() because runtime checks in ccsidcurl.c did not account for conditional directives.

As discussed in pull request #4789, a better way to handle this would be to remove the conditional directives so that the enumeration indices don't change with compiler options, also remove the runtime check and replace with a build time check.

[bagder]

./urldata.h:1437:72: warning: Trailing whitespace (TRAILINGSPACE)
  * null terminated strings MUST be added to the enumeration immediately 
                                                                        ^
./urldata.h:1438:70: warning: Trailing whitespace (TRAILINGSPACE)
  * before STRING_LASTZEROTERMINATED, binary fields immediately before 
                                                                      ^
checksrc: 0 errors and 2 warnings
checksrc: 0 errors and 6 warnings suppressed

[bagder]

Thanks, can you squash this into a single commit and rebase it please? To make it easier to review and see what the ultimate change looks like.

[bagder]

Thanks!