NixOS and Fedora uses .../ca-bundle.crt. Ubuntu uses
.../ca-certificates.crt. Add .../ca-certificates.crt symlink to be
compatible with Ubuntu.
Example use case: Bob has a ~/.msmtprc file that he brings over from
Ubuntu. It also works on NixOS.
the CURL_CA_BUNDLE environment variable. This allows curl to work
without the `-k' flag on https sites with a properly signed
certificate.
svn path=/nixos/trunk/; revision=19572