In many cases, using OpenSSL's default certificate store is fine
and even preferred. If your OpenSSL provider (e.g. your
distribution) is competent, they will manage this database
better than you likely will. With this change, bip will
attempt to use the default certificate store if you set
CA mode but do not specify a certificate store location.
This could be refined to test after enabling the default paths
whether the certificate store is empty, and error/warn if
so.
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