build-sys: Add option to link systemctl statically

build-sys: Add option to link systemctl statically

Systemctl is special because it is required for many tasks that may need to
be performed when the system is not fully configured and/or partially
broken:

1. Installing/Uninstalling services during OS installs and upgrades
2. Shutting down the system

Therefore reduce the number of dependencies that systemctl pulls in, by
not linking to systemd-shared. This brings a bit of resilience to
systemctl (and its aliases shutdown, reboot, etc), by linking against
less external libraries.

Because this extra resilience comes at a cost of approximately 580 KB
extra space, this is done behind a meson build option.