This simplifies unqueuing of downloads and makes it more obvious that
the 'change' being performed is to migrate or replace an old download
handle with a new one.
Use enum connection_state instead of int in load_uri,
proxy_uri, get_proxy_worker, and get_proxy_uri. See commit
d18809522e. I hope that satisfies TCC.
Mostly non-ANSI function declarations, using 0 as NULL and inline
function prototypes. Also removed unused S_HTTP_100 network state
enum type, which text message contained unknown escape sequence: '\?'.
