libobs/util: Improve inhibit functions on Linux

[tytan652]

Description

Avoid creating the screensaver inhibitor thread when another method has been applied.

Make inhibition unavailable in a Flatpak if portal is not available.

Motivation and Context

1. If the Portal or the D-Bus method is in place, adding the xdg-screensaver thread is overkill. It should be just a fallback.
2. Flatpak only support the Portal method, so spamming xdg-screensaver will do nothing

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)
• Performance enhancement (non-breaking change which improves efficiency)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [ ] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.