ITS#6975 MozNSS - allow cacertdir in most cases

OpenLDAP built with OpenSSL allows most any value of cacertdir - directory
is a file, directory does not contain any CA certs, directory does not
exist - users expect if they specify TLS_REQCERT=never, no matter what
the TLS_CACERTDIR setting is, TLS/SSL will just work.
TLS_CACERT, on the other hand, is a hard error.  Even if TLS_REQCERT=never,
if TLS_CACERT is specified and is not a valid CA cert file, TLS/SSL will
fail.  This patch makes CACERT errors hard errors, and makes CACERTDIR
errors "soft" errors.  The code checks CACERT first and, even though
the function will return an error, checks CACERTDIR anyway so that if the
user sets TRACE mode they will get CACERTDIR processing messages.