unified build scheme: give util/dofile.pl the possibility to output selectively

Under certain conditions, one might not want to output certain
sections of a template file.  This adds the functions output_off() and
output_on(), reachable inside the templates.  And example usage in a
Makefile template could be this:

	@ : {- output_off() if $config{no_shared}; "" -}
	... lines dealing with shared libraries
	@ : {- output_on() -}

Reviewed-by: Rich Salz <rsalz@openssl.org>