Use the correct CURLOPT_WRITEFUNCTION signature

[TedLyngmo]

In order to not encourage users to use incompatible function pointers, change the callback function definitions to use char * instead of void * for the first argument.

Triggered by https://stackoverflow.com/questions/79921871/curl-c-c-library-based-application-produces-erronious-response-for-http-post-r#comment141032037_79921871 :
"The code was mostly modified from this example code, honestly I never knew this is wrong. Thanks for pointing it out."

[Copilot]

Pull request overview

This PR updates libcurl tests and documentation/examples to use the canonical CURLOPT_WRITEFUNCTION callback prototype, aligning with curl_write_callback in the public headers and reducing the chance of users copying incompatible function pointer signatures.

Changes:

• Update write callback prototypes from void * to char * for the first argument in docs/examples.
• Adjust a libtest write callback to match the correct callback signature.
• Update tutorial/FAQ text to reflect the correct prototype for user callbacks.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/libtest/lib3102.c	Makes the test write callback match libcurl’s curl_write_callback signature.
docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.md	Fixes the example write callback signature to use char *.
docs/libcurl/libcurl-tutorial.md	Updates the documented write callback prototype (including the C++ example).
docs/FAQ.md	Updates the C++ callback example signature.
docs/examples/usercertinmem.c	Corrects example write callback signature.
docs/examples/url2file.c	Corrects example write callback signature.
docs/examples/synctime.c	Corrects example write callback signature.
docs/examples/sftpget.c	Corrects example write callback signature.
docs/examples/sessioninfo.c	Corrects example write callback signature.
docs/examples/sepheaders.c	Corrects example write callback signature.
docs/examples/postinmemory.c	Corrects example write callback signature.
docs/examples/http2-pushinmemory.c	Corrects example write callback signature.
docs/examples/hiperfifo.c	Corrects example write callback signature.
docs/examples/ghiper.c	Corrects example write callback signature.
docs/examples/getinmemory.c	Corrects example write callback signature.
docs/examples/ftpuploadresume.c	Corrects example write callback signature.
docs/examples/ftpsget.c	Corrects example write callback signature.
docs/examples/ftpget.c	Corrects example write callback signature.
docs/examples/ftp-delete.c	Corrects example write callback signature.
docs/examples/externalsocket.c	Corrects example write callback signature.
docs/examples/evhiperfifo.c	Corrects example write callback signature.
docs/examples/ephiperfifo.c	Corrects example write callback signature.
docs/examples/crawler.c	Corrects example write callback signature.
docs/examples/chkspeed.c	Corrects example write callback signature.
docs/examples/certinfo.c	Corrects example write callback signature.
docs/examples/cacertinmem.c	Corrects example write callback signature.

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[bagder]

Thanks!