MEDIUM: ssl/cli: "dump ssl cert" allow to dump a certificate in PEM format

MEDIUM: ssl/cli: "dump ssl cert" allow to dump a certificate in PEM format

The new "dump ssl cert" CLI command allows to dump a certificate stored
into HAProxy memory. Until now it was only possible to dump the
description of the certificate using "show ssl cert", but with this new
command you can dump the PEM content on the filesystem.

This command is only available on a admin stats socket.

$ echo "@1 dump ssl cert cert.pem" | socat /tmp/master.sock -
-----BEGIN PRIVATE KEY-----
[...]
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
[...]
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
[...]
-----END CERTIFICATE-----