Fix disabling HTTP with ‘cl_http_downloads 0’.

Disable HTTP downloads for entire lifetime of server connection.
Otherwise HTTP_RunDownloads will attempt to queue downloads even if
disabled, resulting in mixed HTTP/UDP downloads and error.
Add client support for compressed UDP downloads.

Support both continuous deflate stream for entire download and chunked
per-packet streams. Add new minor Q2PRO protocol version 1021.
Support reading raw deflated data from *.pkz files.
Add server support for compressed UDP downloads.

Support streaming raw deflated data from *.pkz files to Q2PRO clients.
Don't bother doing any kind of on-the-fly chunked compression.
Remove requirement of setting ‘allow_download_maps’ to 2 to be able to
download maps from packfiles.
Improve server UDP downloading code.

Optimize size of download chunks by filling up whatever buffer space
remains after all reliable messages have been written.
Use proper constant for ‘net_maxmsglen’.
Display client download position in bytes.
Disallow negative ‘scr_conspeed’ values.
Close console after ‘vid_restart’.
Make sure to run CL_Activate after ‘vid_restart’.

Properly release all keys.
Reset fs_gamedir when returning to base game.
Fix position display for resumed download.