Use rb_block_call instead of rb_iterate

[hsbt]

Ruby 4.1.0dev couldn't build curb gem.

curb_easy.c:2660:7: error: call to undeclared function 'rb_iterate'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
 2660 |       rb_iterate(rb_each, rb_easy_get("headers"), cb_each_http_header, wrap);

rb_iterate has been removed from ruby HEAD: https://bugs.ruby-lang.org/issues/21768

I replaced that to rb_block_call.

[Copilot]

Pull request overview

This PR updates the curb gem to be compatible with Ruby 4.1.0dev by replacing the deprecated rb_iterate API with rb_block_call. The rb_iterate function was removed from Ruby HEAD (issue #21768), causing build failures. This change maintains functional equivalence while using the modern Ruby C API.

Changes:

• Replaced 4 occurrences of rb_iterate(rb_each, ...) with rb_block_call(..., rb_intern("each"), ...)
• Updated calls for headers, proxy_headers, ftp_commands, and resolve list iteration

Comments suppressed due to low confidence (4)

ext/curb_easy.c:2248

• The comment states this is an 'rb_iterate callback' but the code now uses rb_block_call. Update the comment to reflect that this is an rb_block_call callback or simply a callback function.

/***********************************************
 * This is an rb_iterate callback used to set up http headers.
 */

ext/curb_easy.c:2287

• The comment states this is an 'rb_iterate callback' but the code now uses rb_block_call. Update the comment to reflect that this is an rb_block_call callback or simply a callback function.

/***********************************************
 * This is an rb_iterate callback used to set up http proxy headers.
 */

ext/curb_easy.c:2323

• The comment states this is an 'rb_iterate callback' but the code now uses rb_block_call. Update the comment to reflect that this is an rb_block_call callback or simply a callback function.

/***********************************************
 * This is an rb_iterate callback used to set up ftp commands.
 */

ext/curb_easy.c:2341

• The comment states this is an 'rb_iterate callback' but the code now uses rb_block_call. Update the comment to reflect that this is an rb_block_call callback or simply a callback function.

/***********************************************
 * This is an rb_iterate callback used to set up the resolve list.
 */

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