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/test985

@@ -0,0 +1,54 @@
+<testcase>
+<info>
+<keywords>
+POP3
+STARTTLS
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<servercmd>
+REPLY CAPA -ERR Not implemented
+</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 require STARTTLS with failing capabilities
+ </name>
+ <command>
+pop3://%HOSTIP:%POP3PORT/%TESTNUMBER -u user:secret --ssl-reqd
+ </command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+# 64 is CURLE_USE_SSL_FAILED
+<errorcode>
+64
+</errorcode>
+<protocol>
+CAPA
+</protocol>
+</verify>
+</testcase>