systemd/systemd: add macros for package install scripts (MR 5377)

These macros are based off of the upstream systemd RPM macros, but it
doesn't implement all of the macro functionality upstream. Initially
they will be used in the install scripts for apkbuilds that provide
systemd service unit files.

For example, a `$pkgname-systemd.post-install` script might have:
    . /usr/lib/systemd/systemd-apk-macros.sh
    systemd_service-post_install system sshd

And a `$pkgname-systemd.pre-deinstall` script might have:
    . /usr/lib/systemd/systemd-apk-macros.sh
    systemd_service_pre_deinstall user pipewire

One day maybe these could be upstreamed to systemd to provide some
apk/abuild compatible macros.

Co-authored-by: jane400 <pmos@j4ne.de>