BUG/MINOR: ssl: fix memory leak at OCSP loading

BUG/MINOR: ssl: fix memory leak at OCSP loading

Fix a memory leak when loading an OCSP file when the file was already
loaded elsewhere in the configuration.

Indeed, if the OCSP file already exists, a useless chunk_dup() will be
done during the load.

To fix it we reverts "ocsp" to "iocsp" like it was done previously.

This was introduced by commit 246c024 ("MINOR: ssl: load the ocsp
in/from the ckch").

Should fix part of the issue #746.

It must be backported in 2.1 and 2.2.