systemd/systemd: rework trigger script

• apk-tools passes the directories it triggers on as arguments to the trigger script. Use them to only run the relevant parts of the triggers.
• Remove "|| :" from commands, as we run the script without "set -e", it won't fail if a single command fails. Put explicit "exit 0" at the end to ensure this even if the last command beforehand failed.
• Print messages for each action so we know what is happening.

Example output:

qemu-amd64:~$ sudo apk add at-spi2-core-systemd
The following NEW packages will be installed:
  at-spi2-core-systemd
Need to download 1689 B of packages.
After this operation, 200 B of additional disk space will be used.
Do you want to continue [Y/n]? 
(1/1) Installing at-spi2-core-systemd (63-r0)
Executing systemd-256.11-r4.trigger
Running systemd daemon-reload...
Running systemd reload user@*.service...
Reloading/restarting marked user services for uid=10000...

As we can see it only runs some of the parts of the trigger script, depending on the dirs that were changed.

Follow-up to !6334 (merged).