Add SSL/TLS encryption in tcpclient

[mbhangui]

This PR adds SSL/TLS encryption in tcpclient. A client program dotls has been included which reads stdin, writes to fd 7 and reads fd 6 and writes to stdout

This PR also adds client mode operation when prog is omitted. In this case tcpclient passes data from fd 0 (stdin) to the network and data from the network to fd 1 (stdout). This makes it work like telnet(1), ssh(1) and other programs

Examples

1. Connect to port 25

   $ tcpclient 0 25
   220 indimail.org (NO UCE) ESMTP IndiMail 1.238 Wed, 3 Mar 2021 18:08:10 +0530
   ehlo
   250-indimail.org [::1]
   250-PIPELINING
   250-8BITMIME
   250-SIZE 10000000
   250-ETRN
   250-STARTTLS
   250 HELP
   quit
   221 indimail.org closing connection

2. Connect to port SMTPS port on 465 using IPv6

   $ tcpclient -n "" ::1 465
   220 indimail.org (NO UCE) ESMTP IndiMail 1.238 Wed, 3 Mar 2021 18:09:52 +0530
   ehlo
   250-indimail.org [::1]
   250-AUTH LOGIN PLAIN CRAM-MD5 CRAM-SHA1 CRAM-SHA256 CRAM-SHA512 CRAM-RIPEMD DIGEST-MD5
   250-AUTH=LOGIN PLAIN CRAM-MD5 CRAM-SHA1 CRAM-SHA256 CRAM-SHA512 CRAM-RIPEMD DIGEST-MD5
   250-PIPELINING
   250-8BITMIME
   250-SIZE 10000000
   250-ETRN
   250 HELP
   quit
   221 indimail.org closing connection

3. Provide oppurtunistic TLS for notqmail/netqmail/qmail qmail-smtpd

   $ tcpserver -HR -v 0 5555 /usr/bin/dotls -n "" -s smtp /var/qmail/bin/qmail-smtpd
   tcpserver: status: 0/40 sql: 1

4. Run tcpclient in client mode and issue STARTTLS command. This is similar to openssl s_client -starttls -crlf

   $ tcpclient -HRv -n "" -s smtp 0 5555
   tcpclient: connected to ::1 port 5555
   220 indimail.org ESMTP
   ehlo
   250-indimail.org
   250-PIPELINING
   250 8BITMIME
   help
   214 notqmail home page: https://notqmail.org
   quit
   221 indimail.org

Files modified tcpclient.c tcpserver.c tcpclient.9 tcpserver.9 Makefile .gitignore TARGETS

Files added tls.c tls.h dotls.c dotls.1