pgsql: In initialize_SSL, don't fail unnecessarily when home dir is una

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 2010-05-26 broke
the case.  Per report from Christian Kastner.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2ff092bcd8e204fa1a8469c64d2cc297242f31f4

Modified Files
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src/interfaces/libpq/fe-secure.c |   64 ++++++++++++++++++++++++-------------
1 files changed, 41 insertions(+), 23 deletions(-)