meson: add a new -Dstandalone-binaries=true option

meson: add a new -Dstandalone-binaries=true option

This adds an option to build standalone binaries that do not depend on the
systemd-shared library. This option can be handy to build binaries that can be
useful on a non-systemd system, binaries such as systemd-sysusers and
systemd-tmpfiles have been previously requested, but installing them with all
the required dependencies pulls in too much code that isn't really relevant for
those use cases. The standalone use case is also relevant in containers, where
minimizing the size of the container image is quite relevant.

For now, only `systemd-sysusers` is also built as a standalone binary.

The standalone binaries are installed as `/usr/bin/%{name}.standalone`, the
packaging system is reponsible for renaming those into the correct names
during the packaging step. RPM is able to do so with RemovePathPostfixes:

The default behavior is to build shared binaries only, since this option is
mainly intended for building distribution packages.

Tested that a proper separate binary is built when using this option and
that having it disabled (or using the default Meson configuration) does not
produce a binary for this option.