ui-shared: add support for NO_HTTP=1/--nohttp

cgit_print_http_headers() used to do nothing if 'embedded' was
specified in cgitrc, but that was wrong - we never want to skip the
headers when invoked as a CGI app. Sadly, there's no easy way to
detect if we're invoked as a CGI app or if we're invoked by another
CGI app, so for the latter case cgit needs to be invoked with either
--nohttp on the command line or NO_HTTP=1 in the environment.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>