Add systemd source code

[tleb]

Sources are hosted here. Tags will need some massaging to fit standard format, not all are in the same format.

Step two could be to support indexing its documentation file format. For example, see doc of ExecCondition=.

[fstachura]

I started working on this here, deployed on https://elixir.fbstc.org/systemd/v256.4/source

Systemd often uses string literal concatenation and it really confuses the lexer, related to #306
Examples:

• https://elixir.fbstc.org/systemd/v256.4/source/src/core/service.c#L225
• https://elixir.fbstc.org/systemd/v256.4/source/src/network/networkd-dhcp4.c#L480
• https://elixir.fbstc.org/systemd/v256.4/source/src/login/logind-dbus.c#L1867
• https://elixir.fbstc.org/systemd/v256.4/source/src/journal/journald-kmsg.c#L71
• https://elixir.fbstc.org/systemd/v256.4/source/src/journal/journald-socket.c#L147