BUG/MEDIUM: proxy: Perform a custom copy for default server settings

BUG/MEDIUM: proxy: Perform a custom copy for default server settings

When a proxy is initialized with the settings of the default proxy, instead
of doing a raw copy of the default server settings, a custom copy is now
performed by calling srv_settings_copy(). This way, all settings will be
really duplicated. Without this deep copy, some pointers are shared between
several servers, leading to UAF, double-free or such bugs.

This patch relies on following commits:

  * b32cb9b51 REORG: server: Export srv_settings_cpy() function
  * 0b365e3cb MINOR: server: Constify source server to copy its settings

This patch should fix the issue #1804. It must be backported as far as 2.0.