proxy: add a free function for the options's pointers

When we duplicate a user-provided options struct, we're stuck with freeing the
url in it. In case we add stuff to the proxy struct, let's add a function in
which to put the logic.

src/proxy.c

@@ -31,3 +31,9 @@ int git_proxy_options_dup(git_proxy_options *tgt, const git_proxy_options *src)
 
 	return 0;
 }
+
+void git_proxy_options_free(git_proxy_options *opts)
+{
+	git__free((char *) opts->url);
+	opts->url = NULL;
+}

src/proxy.h

@@ -12,5 +12,6 @@
 #include "git2/proxy.h"
 
 extern int git_proxy_options_dup(git_proxy_options *tgt, const git_proxy_options *src);
+extern void git_proxy_options_free(git_proxy_options *opts);
 
 #endif