pgsql: Attempt to open certificate file "manually" using fopen before

Log Message:
-----------
Attempt to open certificate file "manually" using fopen before
trying BIO functions.
Helps problem with older versions of OpenSSL that lacks error
stack functions and would show an incorrect error message for
file-not-found-or-not-openable. The problem may still exist for
other errors, but file open error is by far the most common one.

Modified Files:
--------------
pgsql/src/interfaces/libpq:
fe-secure.c (r1.97 -> r1.98)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-secure.c?r1=1.97&r2=1.98)