Add __attribute__((const, pure, format)) in various places

I'm assuming that it's fine if a _const_ or _pure_ function
calls assert. It is assumed that the assert won't trigger,
and even if it does, it can only trigger on the first call
with a given set of parameters, and we don't care if the
compiler moves the order of calls.
time-dst: use _cleanup_
build-sys: "link" python _reader and pam_systemd against libsd-daemon-internal

The same old story as d3b9e0ff: those two use libsystemd-shared, and
in turn, some functions in libsystemd-shared use libsystemd-daemon.
The fact that *those* functions are used neither by the python modules
in question nor pam_systemd isn't always enough. Currently, I'm seeing
linking failures with -flto. The result of adding
libsystemd-daemon-internal to the list of linked libraries should be
harmless, with no change in size or final link requirements.
man: link to socket activation blog entries