lib: move mimepost data from ->req.p.http to ->state

lib: move mimepost data from ->req.p.http to ->state

When the legacy CURLOPT_HTTPPOST option is used, it gets converted into
the modem mimpost struct at first use. This data is (now) kept for the
entire transfer and not only per single HTTP request. This re-enables
rewind in the beginning of the second request instead of in end of the
first, as brought by 1b39731.

The request struct is per-request data only.

Extend test 650 to verify.

Fixes #11680
Reported-by: yushicheng7788 on github
Closes #11682