In initialize_SSL, don't fail unnecessarily when home dir is unavailable.
Instead, just act as though the certificate file(s) are not present.
There is only one case where this need be a hard failure condition: when
sslmode is verify-ca or verify-full, not having a root cert file is an
error. Change the logic so that we complain only in that case, and
otherwise fall through cleanly. This is how it used to behave pre-9.0,
but my patch
4ed4b6c54e5fab24ab2624d80e26f7546edc88ad of 2010年05月26日 broke
the case. Per report from Christian Kastner.