MEDIUM: proxy: store the default proxies in a tree by name

MEDIUM: proxy: store the default proxies in a tree by name

Now default proxies are stored into a dedicated tree, sorted by name.
Only unnamed entries are not kept upon new section creation. The very
first call to cfg_parse_listen() will automatically allocate a dummy
defaults section which corresponds to the previous static one, since
the code requires to have one at a few places.

The first immediately visible benefit is that it allows to reuse
alloc_new_proxy() to allocate a defaults section instead of doing it by
hand. And the secret goal is to allow to keep multiple named defaults
section in memory to reuse them from various proxies.