Fixes Redmine #1855: don't close file descriptors we use

I've been studying the Redmine #1855 bug, which causes promise analysis strings (mostly HTML, sometimes TEXT) to be output to syslog at random times.

I've included a full explanation on https://cfengine.com/dev/issues/1855, but basically this patch gets the file descriptors used to write the HTML/TEXT promise analysis strings to files (actually, /dev/null usually) and makes sure the ActAsDaemon function doesn't close them. I believe that closing them, then writing to them, is what's causing this issue.

I apologize if this patch doesn't match CFEngine coding style or best practices - I did my best to match what was around in the file. Also, if I'm screwing up in my logic here, please let me know. Basically just tell me how you feel about this and I'll rework it if necessary.