This includes setting the new priority and adding the download to the list
of connection downloads. If the connection has no downloads set the
PRI_CANCEL priority; get_priority() requires that.
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
d18809522ed7242ac9bfaab9a72a0b23b9d46f1c. 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: '\?'.
