[mod_cgi] simplify mod_cgi_handle_subrequest()

[mod_cgi] simplify mod_cgi_handle_subrequest()

wait for CGI to close stdout, so we read EOF on pipe to end CGI response

remove extra call to waitpid() which will occur after process exits
if it has not already been explicitly closed by CGI (and has not been
inherited by CGI forked children)  (If CGI forks, then it should close
its stdout response pipe when response is done, especially if it intends
to perform lengthy post-processing in the background.)

From: Glenn Strauss <gstrauss@gluelogic.com>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@3091 152afb58-edef-0310-8abb-c4023f1b3aa9