Add meson build option for disabling installing systemd service

[hendursaga]

As title says. The relevant line is https://github.com/alexr4535/siglo/blob/main/data/meson.build#L21 - I've never used meson/ninja before, though. My reason is because I have no need for the service and also meson was asking polkit for elevated permissions without telling really me why.

[eli-schwartz]

Seems like the project should probably just install to join_paths(get_option('prefix'), 'lib/systemd/user') instead. Hardcoding /etc is sub-optimal (systemd prefers /usr/lib/ but if you ever do need /etc/ you should still use get_option('sysconfdir')).

[hendursaga]

@eli-schwartz sounds like an improvement. At the very least I'd like to see a more helpful permissions request..