Refactor the sslfiles Makefile target for ease of use

Refactor the sslfiles Makefile target for ease of use

The Makefile handling of certificate and keypairs used for TLS testing
had become quite difficult to work with. Adding a new cert without the
need to regenerate everything was too complicated. This patch refactors
the sslfiles make target such that adding a new certificate requires
only adding a .config file, adding it to the top of the Makefile, and
running make sslfiles.

Improvements:
- Interfile dependencies should be fixed, with the exception of the CRL
  dirs.
- New certificates have serial numbers based on the current time,
  reducing the chance of collision.
- The CA index state is created on demand and cleaned up automatically
  at the end of the Make run.
- *.config files are now self-contained; one certificate needs one
  config file instead of two.
- Duplication is reduced, and along with it some unneeded code (and
  possible copy-paste errors).
- all configuration files underneath the conf/ directory.

The target is moved to its own makefile in order to avoid colliding
with global make settings.

Author: Jacob Champion <pchampion@vmware.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/d15a9838344ba090e09fd866abf913584ea19fb7.camel@vmware.com