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.

This PR has the same fix as norihiro/obs-command-source#34.

How Has This Been Tested?

Tested on Linux.

• Checked file descriptors opened by OBS are not inherited in the sub process.
• Checked the audio arrives to OBS as usual.

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.

Fix #17