SImplify the functionality of script_concat_settings() and remove the globals

[azaozz]

As discussed in the comments on #20055, script_concat_settings() should be split into couple of simple functions that only check the constants and the 'can_compress_scripts' server option.

There's an initial patch by ericlewis: http://core.trac.wordpress.org/attachment/ticket/20055/20055-3.patch

Not sure what the best back-compat option is here. Outright removing the globals would break plugins that use them. Seems we should move script_concat_settings() to deprecated.php and trigger the appropriate "deprecated" messages.

Then we can introduce simplified function that would return whether compression from PHP is supported by the server using CONCATENATE_SCRIPTS, COMPRESS_SCRIPTS and get_site_option('can_compress_scripts').

This didn't make it in 3.4.