handle request automatically on PLP::CGI import

handle request automatically on PLP::CGI import

As it's not used for much else, PLP::CGI can simply start running on
import. So like PLP::FastCGI, the executable only needs to C<use PLP::CGI>
(and C<require> for advanced usage).

No backwards compatibility issues, as the last release didn't have this
modularized (PLP wrapper works the same as before).