Added new cl_hooks, menu_cmd isdemo and fixed demoseeking.cfg

This branch documents cl_hook_game* implementation with some comments and adds start and end hooks for demos as those were missing and game hooks were not fired during demos.

Hook explanations:

• cl_hook_gamestart hook is still only fired when a gamemode is init'd or with nop fallback if that hasn't happened before VM shutdown

• cl_hook_gameend hook now with a breaking change no longer fires at the start of an intermission if a gamestart hook has been called OR when the CSQC VM shuts down. It is now always only called when the CSQC VM shuts down

• cl_hook_intermission is now a separate hook from cl_hook_gameend which is only fired when an intermission starts if a gamestart hook has been called

• a new cl_hook_demostart hook is now fired when a gamemode is properly init'd while playing a demo

• a new cl_hook_demointermission hook is now fired if a gamemode has been properly init'd and an intermission starts running in a demo

• a new cl_hook_demoend hook is now fired when the CSQC VM shuts down while playing a demo

New menu_cmd isdemo

Now there is a way to live check if your client is running a demo or not from the console. If menu_cmd isdemo is called without an argument then it will create a new cvar _menu_isdemo while if it called with an argument like set asdf "69";menu_cmd isdemo asdf then it'll write into asdf either a 1 or a 0 depending on if a demo is playing or not.

Breaking changes:

None anymore.

Backwards compatibility issues:

Old servers and old demos will never have cl_hook_demo* hooks. This means that they can not be used for any fancy stuff as they depend on demo age and servers to update.

Fixes:

Reworked how demos are detected to allow forward seeking in demos for engine master branch clients with new demos. When darkplaces#397 (closed) is fixed master engine branch is able to seek backwards too without manually setting _current_demo_name "demos/asdf.dem"

Cruft Removal:

• _demo_is_playing variable is no longer needed at all AFAIK and thus it was removed
• alias cl_hook_gamestart_all "demoseeking_game_started" is no longer needed to be set by the player in their configs
• _demoseeking_playdemo_check hack which depended on weird behavior which changed in master branch which broke the hack