MINOR: ssl: diagnostic warning when both 'default-crt' and 'strict-sni' are used

MINOR: ssl: diagnostic warning when both 'default-crt' and 'strict-sni' are used

It possible to use both 'strict-sni' and 'default-crt' on the same bind
line, which does not make much sense.

This patch implements a check which will look for default certificates
in the sni_w tree when strict-sni is used. (Referenced by their empty
sni ""). default-crt sets the CKCH_INST_EXPL_DEFAULT flag in
ckch_inst->is_default, so its possible to differenciate explicits
default from implicit default.

Could be backported as far as 3.0.

This was discussed in ticket #3082.