lib: stop overriding system printf symbols

After this patch, the codebase no longer overrides system printf
functions. Instead it explicitly calls either the curl printf functions
`curl_m*printf()` or the system ones using their original names.

Also:
- drop unused `curl_printf.h` includes.
- checksrc: ban system printf functions, allow where necessary.

Follow-up to db98daab05 #18844
Follow-up to 4deea9396b #18814

Closes #18866

lib/imap.c

@@ -78,8 +78,7 @@
 #include "curlx/warnless.h"
 #include "curl_ctype.h"
 
-/* The last 3 #include files should be in this order */
-#include "curl_printf.h"
+/* The last 2 #include files should be in this order */
 #include "curl_memory.h"
 #include "memdebug.h"
 
@@ -1943,9 +1942,9 @@ static CURLcode imap_sendf(struct Curl_easy *data,
   DEBUGASSERT(fmt);
 
   /* Calculate the tag based on the connection ID and command ID */
-  msnprintf(imapc->resptag, sizeof(imapc->resptag), "%c%03d",
-            'A' + curlx_sltosi((long)(data->conn->connection_id % 26)),
-            ++imapc->cmdid);
+  curl_msnprintf(imapc->resptag, sizeof(imapc->resptag), "%c%03d",
+                 'A' + curlx_sltosi((long)(data->conn->connection_id % 26)),
+                 ++imapc->cmdid);
 
   /* start with a blank buffer */
   curlx_dyn_reset(&imapc->dyn);