MINOR: ssl: 'ssl-load-extra-del-ext' removes the certificate extension

MINOR: ssl: 'ssl-load-extra-del-ext' removes the certificate extension

In issue #785, users are reporting that it's not convenient to load a
".crt.key" when the configuration contains a ".crt".

This option allows to remove the extension of the certificate before
trying to load any extra SSL file (.key, .ocsp, .sctl, .issuer etc.)

The patch changes a little bit the way ssl_sock_load_files_into_ckch()
looks for the file.