MEDIUM: ssl: generate '*' SNI filters for default certificates

MEDIUM: ssl: generate '*' SNI filters for default certificates

This patch follows the previous one about default certificate selection
("MEDIUM: ssl: allow multiple fallback certificate to allow ECDSA/RSA
selection").

This patch generates '*" SNI filters for the first certificate of a
bind line, it will be used to match default certificates. Instead of
setting the default_ctx pointer in the bind line.

Since the filters are in the SNI tree, it allows to have multiple
default certificate and restore the ecdsa/rsa selection with a
multi-cert bundle.

This configuration:
   # foobar.pem.ecdsa and foobar.pem.rsa
   bind *:8443 ssl crt foobar.pem crt next.pem

will use "foobar.pem.ecdsa" and "foobar.pem.rsa" as default
certificates.

Note: there is still cleanup needed around default_ctx.

This was discussed in github issue #2392.