ftp,imap,pop3: do not ignore --ssl-reqd

In imap and pop3, check if TLS is required even when capabilities
request has failed.

In ftp, ignore preauthentication (230 status of server greeting) if TLS
is required.

Bug: https://curl.se/docs/CVE-2021-22946.html

CVE-2021-22946

tests/data/test986

@@ -0,0 +1,53 @@
+<testcase>
+<info>
+<keywords>
+FTP
+STARTTLS
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<servercmd>
+REPLY welcome 230 Welcome
+REPLY AUTH 500 unknown command
+</servercmd>
+</reply>
+
+# Client-side
+<client>
+<features>
+SSL
+</features>
+<server>
+ftp
+</server>
+ <name>
+FTP require STARTTLS while preauthenticated
+ </name>
+<file name="log/test%TESTNUMBER.txt">
+data
+    to
+      see
+that FTPS
+works
+  so does it?
+</file>
+ <command>
+--ssl-reqd --ftp-ssl-control ftp://%HOSTIP:%FTPPORT/%TESTNUMBER -T log/test%TESTNUMBER.txt -u user:secret
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+# 64 is CURLE_USE_SSL_FAILED
+<errorcode>
+64
+</errorcode>
+<protocol>
+AUTH SSL
+AUTH TLS
+</protocol>
+</verify>
+</testcase>