configure libxmlsec to not spew stuff to stdout

[crewjam]

example:

func=xmlSecOpenSSLAppCertLoadBIO:file=app.c:line=1254:obj=unknown:subj=PEM_read_bio_X509_AUX:error=4:crypto library function failed:
func=xmlSecOpenSSLAppKeyFromCertLoadBIO:file=app.c:line=924:obj=unknown:subj=xmlSecOpenSSLAppCertLoadBIO:error=1:xmlsec library function failed:
func=xmlSecOpenSSLAppKeyLoadBIO:file=app.c:line=334:obj=unknown:subj=xmlSecOpenSSLAppKeyFromCertLoadBIO:error=1:xmlsec library function failed:
func=xmlSecOpenSSLAppKeyLoadMemory:file=app.c:line=205:obj=unknown:subj=xmlSecOpenSSLAppKeyLoadBIO:error=1:xmlsec library function failed:

[crewjam]

this is partially complete. we've captured the libxmlsec errors, but libxml2 still emits errors to stdout that we do not capture