Ensure systemd-resolved has mDNS enabled

[elyscape]

♻️ Current situation

Per #980, the systemd-resolved advertiser reports that it's available even if systemd-resolved has mDNS disabled.

💡 Proposed solution

Check that mDNS is enabled in systemd-resolved before reporting that it is available as an advertiser.

⚙️ Release Notes

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Testing

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Reviewer Nudging

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Notes

This also adds a note to the error message when advertisements fail that there may be a permissions issue and the user should check the wiki for more details. The info to be added to the wiki will be posted in #980.

[elyscape]

cc: @Supereg

[coveralls]

Pull Request Test Coverage Report for Build 3374361088

• 2 of 14 (14.29%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 51.868%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib/Advertiser.ts	2	14	14.29%

Files with Coverage Reduction	New Missed Lines	%
src/lib/Advertiser.ts	1	40.0%

Totals
Change from base Build 3370777112:	-0.05%
Covered Lines:	5821
Relevant Lines:	10259

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3417210565

• 6 of 31 (19.35%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 51.815%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib/Advertiser.ts	6	31	19.35%

Files with Coverage Reduction	New Missed Lines	%
src/lib/Advertiser.ts	1	38.44%

Totals
Change from base Build 3370777112:	-0.1%
Covered Lines:	5824
Relevant Lines:	10271

---

💛 - Coveralls

[Supereg]

Thanks a lot for dealing with this so fast 🚀

Looks good to me. Only had two small comments.

[Supereg]

Just released @homebridge/dbus-native v0.5.0 with the respective change implemented in homebridge/dbus-native#2

[elyscape]

Okay, this should be good to go now.

[Supereg]

Thanks a lot 🚀