Close non-standard file descriptors

[norihiro]

Description

The obs-ffmpeg-mux process cannot be terminated while the process forked by this plugin is running, which causes audio subsystem of OBS stalls. The obs-ffmpeg-mux process is terminated when an EOF appears on the pipe. However, since the forked process holds the file descriptor of the write-end of the pipe, the EOF won't arrive to the obs-ffmpeg-mux process.

How Has This Been Tested?

Tested on Linux and macOS.

1. Start recording.
2. Trigger a command sleep 60 from this plugin.
3. Stop recording.
4. Check the recording stops immediately. (before this fix, it takes 1 minutes because there is sleep 1m).

General checklist

• The commit is reviewed by yourself.
• The code is tested.
• Document is up to date or not necessary to be changed.
• The commit is compatible with repository's license.