merge: improvements to libconn

 * replace mutt_socket_write_d by mutt_socket_send_d
 * rename mutt_socket_write to mutt_socket_send
 * introduce mutt_socket_{read,write}
 * don't try to determine buffer size if length was negative
 * rename socket_new_conn to mutt_socket_new
 * define mutt_ssl_socket_setup even when building without SSL
 * move mutt_socket_free to conn/socket.c
 * let mutt_socket_new add Connection to the list
 * move Connections declaration to conn/socket.c
 * add parameter to mutt_socket_new to specify socket type
 * move raw connection to its own file
 * conn/ssl.c: rename local var data to ssldata
 * factorise ssl_socket_open and mutt_ssl_starttls
 * rename tls_close to ssl_socket_close_and_restore