[PATCH 3 of 3] [RFC] sslutil: try to find and use system CA file when appropriate

[timeless]

One thing we could do is abort but spit out a config that would work.

So if we detect:
/etc/pki/tls/certs/ca-bundle.trust.crt

abort: ca bundle isn't configured
you can add the following to .hg/hgrc:
web.cacerts=/etc/pki/tls/certs/ca-bundle.trust.crt