feature: config file, config cvar and forced cvars for plugin settings, Deathmatch weapon rotation

[lukepolo]

A registry entry can describe a plugin's config file and the cvars it forces, and Deathmatch now does.

• New fields config_cvar, config_default, config_shipped and forced_cvars, validated, with the shipped default checked against config_schema (including duplicate entries, which make Deathmatch drop the whole file)
• Deathmatch: weapon rotation schema and default, dm_modes_file as its config cvar, and the 107 cvars it resets on every map