Test for /etc/ssl/cert.pem existence to avoid masking SSL_CA_CERT_PATH

Test for /etc/ssl/cert.pem existence to avoid masking SSL_CA_CERT_PATH

Prior to this patch, unless SSL_CA_CERT_FILE is set in the environment,
libfetch will set the CA file to "/usr/local/etc/cert.pem" if it exists,
and to "/etc/ssl/cert.pem" otherwise. This has the consequence of
masking SSL_CA_CERT_PATH, because OpenSSL will ignore the CA path if a CA
file is set but fails to load (see X509_STORE_load_locations()).

While here, fall back to OpenSSL defaults if neither SSL_CA_CERT_FILE nor
SSL_CA_CERT_PATH are set in the environment, and if neither of the
libfetch default CA files exists.

PR:		193871
Submitted by:	John W. O'Brien <john@saltant.com>
Approved by:	des
MFC after:	1 week