etc: UPower.conf.d: add UPower.conf.d

The configuration files in the UPower.conf.d will override the primary
configuration.

etc/UPower.conf.d/README.md

@@ -0,0 +1,83 @@
+# UPower Configuration Override
+
+The configuration in UPower.conf.d will override the primary configuration
+in UPower.conf.
+
+The primary method for overriding settings in the main UPower.conf file is by placing configuration snippets in the `/etc/UPower/UPower.conf.d/` directory.
+
+## The format of filename
+
+For a file to be correctly processed as an override, its filename must adhere to a strict format:
+
+- It must start with a two-digit number between `00` and `99`.
+- It must end with the `.conf` extension.
+- The middle section can contain `alphanumeric characters`, `dashes`, and `underscores`.
+
+This format is captured by the regular expression `^([0-9][0-9])-([a-zA-Z0-9-_])*\.conf$`.
+
+- The valid examples
+
+```text
+01-upower-example.conf
+02-upower-test-123.conf
+03-upower-1a2b-3c4D__.conf
+```
+
+- The invalid examples
+
+```text
+0000-upower-abcd1234.conf
+001-upower-@bcd.config
+```
+
+## The configuration override
+
+UPower processes configuration files in sorted order, where settings in later files override identical settings (matching both Group and Key) from previous files, including the main UPower.conf. This hierarchy ensures that local, numerically-prefixed files in the drop-in directory (UPower.conf.d/) take precedence.
+
+For example, consider `UPower.conf` that contains the defaults:
+
+```text
+PercentageLow=20.0
+PercentageCritical=5.0
+PercentageAction=2.0
+```
+
+and there is a file `UPower.conf.d/70-change-percentages.conf`
+containing settings for all `Percentage*` keys:
+
+```text
+[UPower]
+PercentageLow=15.0
+PercentageCritical=10.0
+PercentageAction=5.0
+```
+
+and another `UPower.conf.d/99-change-percentages-local.conf`
+containing settings only for `PercentageAction`:
+
+```text
+[UPower]
+PercentageAction=7.5
+```
+
+First the main `UPower.conf` will be processed, then
+`UPower.conf.d/70-change-percentages.conf` overriding the defaults
+of all percentages from the main config file with the given values,
+and finally `UPower.conf.d/99-change-percentages-local.conf`
+overriding once again only `PercentageAction`. The final, effective
+values are:
+
+```text
+PercentageLow=15.0
+PercentageCritical=10.0
+PercentageAction=7.5
+```
+
+## The examples of overriding the default percentage policy values
+
+```text
+[UPower]
+PercentageLow=25.0
+PercentageCritical=10.0
+PercentageAction=5.0
+```

etc/UPower.conf.d/meson.build

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+install_data(
+    'README.md',
+    install_tag: 'doc',
+    install_dir: get_option('sysconfdir') / 'UPower' / 'UPower.conf.d',
+)

etc/meson.build

@@ -1 +1,2 @@
+subdir('UPower.conf.d')
 install_data('UPower.conf', install_dir: get_option('sysconfdir') / 'UPower')