MINOR: proxy: keep default-server unless empty setting

MINOR: proxy: keep default-server unless empty setting

Previously, default-server were removed as soon as a proxy section
parsing was over. This patch changes this behavior to now keep
default-server during haproxy runtime. This will allow to reuse them for
dynamic servers, which will be later implemented via "from" keyword as a
distinct feature.

Default-server may consume a noticeable amount of memory, so it can
still be desirable to clean up these elements on post-parsing. Thus, if
a default-server is reset, it will still be purged on section post
parsing. This can be achived by using a "default-server from none" final
line.

To detect if a default-server should be preserved, a new flag
SRV_F_UMODIFIED has been defined. Such flag is set as soon as a server
keyword has been parsed. It is reset when srv_settings_init() is used.
Thus, if this flag is present, default-server purge on post parsing is
skipped.