Unnecessary APIs have been removed from nm-setting-proxy, client like
nm-connection-editor are expected to create a PAC script snippet the load
the location of file in NM.
The API of NMProxyConfig exposes @proxies and @excludes as strv values.
There is no need to track those values internally as a GPtrArray and
then clone them in the getters (especially, since the entire NMProxyConfig
API is internal to core.
Thereby, fix a few memory leaks in add_proxy_config() and some
style fixes for { }.
