Implement API function "Host_Pause"

[jonathan-up]

If need to pause, do not need to set pausable to 1 and then make client execute pause command anymore.
This stuff make pause easier and fix some clients may block pause commands sent by the server.

[rtxa]

Thanks for implementing this, it would benefit AG and AG Mod X because (in my case) I rely on a random user to pause/unpause the server, which is odd and unrealiable. Maybe a bit offtopic, but did you take a look to this issue? #862 Unfortunately ReHLDS stops sending server info when on pause and makes players think the server is down, which is not the case in HLDS.

[jonathan-up]

but did you take a look to this issue? #862
@rtxa I reply you in detail in that issues