Murmur: Replace all default CA certs if requiring CA match.

In the interest of ensuring security, Murmur should replace the entire
default CA list with just the one specified in the config file and
any included with the server cert.

Anyone using murmur with strict CA verification has a slim to none
chance of actually wanting all defaultCA certs to be trusted by their
server - if for some reason they do, they can (and indeed, SHOULD)
bundle the relevant ones into the file manually.