MINOR: proxy: add proxy_free_common() helper function

MINOR: proxy: add proxy_free_common() helper function

As shown by previous patch series, having to free some common proxy
struct members twice (in free_proxy() and proxy_free_defaults()) is
error-prone: we often overlook one of the two free locations when
adding new features.

To prevent such bugs from being introduced in the future, and also avoid
code duplication, we now have a proxy_free_common() function to free all
proxy struct members that are common to all proxy types (either regular or
default ones).

This should greatly improve code maintenance related to proxy freeing
logic.