MINOR: ssl: add the ssl_fc_crtname sample fetch

MINOR: ssl: add the ssl_fc_crtname sample fetch

This new sample fetch returns the name of the certificate selected for
an incoming SSL/TLS connection, as it would appear in "show ssl cert".
It may be a filename with its relative or absolute path, or an alias,
depending on how the certificate was declared in the configuration.

The certificate name is stored as ex_data on the SSL_CTX at load time
in ckch_inst_new_load_store(), and freed via a dedicated free callback.