It would be great to have a items witch provide the availability to check the ssl connectivity of ssl-enabled services (HTTPS, LDAPS, IMAPS, ....)

I recommend the following syntax:

Item No.1 : net.tcp.service.ssl.certificate[<type>,<ip>,<port>]

type - i.e. SSL, TLS, ...
ip - IP address (default is 127.0.0.1)
port - port number (by default standard service port number is used)

Return Value:
0 => Unable to establish a ssl/tls connection (ssl connect failure - i.e expired certificate)
>0 => Connection was successful, but certificate expires in returned number of days

Example : net.tcp.service.ssl[ssl,www.verisign.com,443]

Item No.2 : net.tcp.service.ssl.connect[<type>,<ip>,<port>]

type - i.e. SSL, TLS, ...
ip - IP address (default is 127.0.0.1)
port - port number (by default standard service port number is used)

Return the error code:
=> see - http://en.wikipedia.org/wiki/Transport_Layer_Security#Alert_protocol
=> provide a value mapping in Zabbix

see also: ZBXNEXT-287