crypto/x509: add /etc/ssl/certs to certificate directories

[deafgoat]

This will allow certificate verification from system root CAs work for distros such as SLES10/SLES11 which have certificates stored there.

Originally asked about this in https://groups.google.com/forum/#!topic/golang-nuts/lEInOWNOVxk

[rsc]

It is OK to add /etc/ssl/certs to crypto/x509/root_unix.go.