BUG/MINOR: server: do not leak default-server in defaults sections

BUG/MINOR: server: do not leak default-server in defaults sections

When a default-server directive is used in a defaults section, it's never
freed and the "defaults" proxy gets reset without freeing the fields from
that default-server. Normally there are no allocation there, except for
the config file location stored in srv->conf.file form an strdup() since
commit 9394a9444 ("REORG: server: move alert traces in parse_server")
that appeared in 2.4. In addition, if a "default-server" directive
appears multiple times in a defaults section, one more entry will be
leaked per call.

This commit addresses this by checking that we don't overwrite the file
upon multiple calls, and by clearing it when resetting the default proxy.
This should be backported to 2.4.