ftp,imap,pop3,smtp: reject STARTTLS server response pipelining

If a server pipelines future responses within the STARTTLS response, the
former are preserved in the pingpong cache across TLS negotiation and
used as responses to the encrypted commands.

This fix detects pipelined STARTTLS responses and rejects them with an
error.

CVE-2021-22947

Bug: https://curl.se/docs/CVE-2021-22947.html

tests/data/test982

@@ -0,0 +1,57 @@
+<testcase>
+<info>
+<keywords>
+POP3
+STARTTLS
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<servercmd>
+CAPA STLS USER
+REPLY STLS -ERR currently unavailable\r\n+OK user accepted\r\n+OK authenticated
+REPLY PASS -ERR Authentication credentials invalid
+</servercmd>
+<data nocheck="yes">
+From: me@somewhere
+To: fake@nowhere
+
+body
+
+--
+  yours sincerely
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<features>
+SSL
+</features>
+<server>
+pop3
+</server>
+ <name>
+POP3 STARTTLS pipelined server response
+ </name>
+ <command>
+pop3://%HOSTIP:%POP3PORT/%TESTNUMBER -u user:secret --ssl
+ </command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+# 8 is CURLE_WEIRD_SERVER_REPLY
+<errorcode>
+8
+</errorcode>
+<protocol>
+CAPA
+STLS
+</protocol>
+</verify>
+</testcase>