sendf: change Curl_read_plain to wrap Curl_recv_plain

sendf: change Curl_read_plain to wrap Curl_recv_plain

Prior to this change Curl_read_plain would attempt to read the
socket directly. On Windows that's a problem because recv data may be
cached by libcurl and that data is only drained using Curl_recv_plain.

Rather than rewrite Curl_read_plain to handle cached recv data, I
changed it to wrap Curl_recv_plain, in much the same way that
Curl_write_plain already wraps Curl_send_plain.

Curl_read_plain -> Curl_recv_plain
Curl_write_plain -> Curl_send_plain

This fixes a bug in the schannel backend where decryption of arbitrary
TLS records fails because cached recv data is never drained. We send
data (TLS records formed by Schannel) using Curl_write_plain, which
calls Curl_send_plain, and that may do a recv-before-send
("pre-receive") to cache received data. The code calls Curl_read_plain
to read data (TLS records from the server), which prior to this change
did not call Curl_recv_plain and therefore cached recv data wasn't
retrieved, resulting in malformed TLS records and decryption failure
(SEC_E_DECRYPT_FAILURE).

The bug has only been observed during Schannel TLS 1.3 handshakes. Refer
to the issue and PR for more information.

Ref: https://github.com/curl/curl/issues/9431#issuecomment-1312420361

Assisted-by: Joel Depooter
Reported-by: Egor Pugin
Fixes https://github.com/curl/curl/issues/9431
Closes https://github.com/curl/curl/pull/9904