profiles: nextcloud: add dbus-user.own & document dbus service

[glitsj16]

Relates to:

• nextcloud: program escapes dbus-proxy via systemd #6209

Additional context:

• Created wiki entry on how to create local D-Bus overrides
• Edited the firecfg FAQ to make it clear there's native support via firecfg.d now

[rusty-snake]

Technically approved.

[rusty-snake]

Actually this PR weakens the D-Bus filter.

[glitsj16]

@rusty-snake

• changed the PR title to better express what it does
• I'll revisit the wiki entrees ASAP

Thanks for your enlightening comments!

[kmk3]

@glitsj16

Please use "squash and merge" by default again on future profile-only PRs, as
most of them are about a single logical change (and many of them contain
multiple amend/fixup commits, which add noise).

For example, this is the (simplified) git log of the recent relevant merges:

$ git log --oneline --graph --no-decorate f7e2c25730..44e241261e
* 44e241261 electron-cash: use new private-etc syntax
*   7ab8d45dc Merge pull request #6211 from glitsj16/nextcloud-fix
|\
| * b97e853a3 nextcloud: fix the comment link to the wiki FAQ
| * bc3968bb2 nextcloud: edit dbus comment as requested in review
| * 15fc09ec7 nextcloud: harden D-Bus filtering
*   2bca27a10 Merge pull request #6181 from haplo/electron-cash
|\
| * f0cfc70cc Blacklist ~/.electron-cash in disable-programs.inc
| * dae8e6991 electron-cash.profile
* 5954f72e9 Merge pull request #6180 from haplo/rawtherapee
* 84a5f1a60 rawtherapee.profile
$ git log --oneline --graph --no-decorate f7e2c25730..44e241261e | wc -l
12

With squashing, it would look something like this:

$ git log --oneline --graph --no-decorate aaaaaaaaaa~1..cccccccccc
* ccccccccc Profile for Electron Cash (#6181)
* bbbbbbbbb nextcloud: D-Bus filtering changes (#6211)
* aaaaaaaaa Profile for RawTherapee (#6180)
$ git log --oneline --graph --no-decorate aaaaaaaaaa~1..cccccccccc | wc -l
3

When in doubt feel free to ping me.

Related discussion: #6201

[glitsj16]

@kmk3

This flew under my radar before. Thanks for reminding me. I've just merged my wusc-fixes. Hope I got it right this time...