Fix MVD client skipping removed but seen entities.

Fix a few places where num_edicts limit was incorrectly used.
Mark all seen entities with a SVF_ flag.
Improve MVD chasecam, add ‘autochase’ command.

When current chase target becomes inactive, jump to the next available
target.

Automatically return to the previous chase target after map change if
player spawns after spectator enters the map.

Preserve all chasecam parameters between map changes.

Add ‘autochase’ command that restricts subset of players that can be
chased and automatically switches to the next player from this subset
when current chase target becomes inactive. Wildcards are supported for
adding and removing players.