BUG/MINOR: server: Don't rely on last default-server to init server SSL context

BUG/MINOR: server: Don't rely on last default-server to init server SSL context

During post-parsing stage, the SSL context of a server is initialized if SSL
is configured on the server or its default-server. It is required to be able
to enable SSL at runtime. However a regression was introduced, because the
last parsed default-server is used. But it is not necessarily the
default-server line used to configure the server. This may lead to
erroneously initialize the SSL context for a server without SSL parameter or
the skip it while it should be done.

The problem is the default-server used to configure a server is not saved
during configuration parsing. So, the information is lost during the
post-parsing. To fix the bug, the SRV_F_DEFSRV_USE_SSL flag is
introduced. It is used to know when a server was initialized with a
default-server using SSL.

For the record, the commit f63704488e ("MEDIUM: cli/ssl: configure ssl on
server at runtime") has introduced the bug.

This patch must be backported as far as 2.4.