Always load CA, key and certificate files at the time the configuration

Always load CA, key and certificate files at the time the configuration
function is called. This simplifies code and results in a single memory
based code path being used to provide data to libssl. Errors that occur
when accessing the specified file are now detected and propagated
immediately. Since the file access now occurs when the configuration
function is called, we now play nicely with privsep/pledge.

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