This commit uses the abstractions added in the previous commit to
add a bunch more properties to the io.systemd.StartTransient()
to showcase how straightforward this is now.
New helpers for tristate bools and an init helper are added. A
dedicated dispatcher for LogLevelMax parses the string-form name
("info", "debug" etc.) declared in the varlink IDL.
The new properties are: DynamicUser, IgnoreSIGPIPE, LockPersonality,
MemoryDenyWriteExecute, NoNewPrivileges, OOMScoreAdjust, RemoveIPC,
RestrictRealtime, RestrictSUIDSGID, RootEphemeral, UMask.
The remaining ProtectKernel*, Private*, ProtectClock properties are
declared as STRING in the varlink IDL (matching the modern *Ex/enum
form) so a bool dispatcher does not pass schema validation. Those
need a string-parsing dispatcher and will be added in a follow-up.
This brings us closer to parity with the D-Bus code (still a long
way to go though).