plugins: capture total module count

...in a global variable, alongside the plugin list and such.

this allows us to improve the efficiency of all module_list_get() calls since we know ahead of time what the total size of the array needs to be.

additionally, the module_LoadPlugins() function was making a wildly inefficient call to that function, relatively speaking, just to be able to print the count in a debug message. this can also use this count directly to avoid the unnecessary work.