pkg-config: export systemd{system,user}generatordir and catalogdir

We export the location of a bunch of directories this way,
so it makes sense to add those three. Especially catalogdir
is something that we want people to add things to.

Note on the naming: the first two are tied closely to systemd
itself, so I prefixed them with "systemd". The third one is
rather more generic, so no prefix.

https://bugs.freedesktop.org/show_bug.cgi?id=67635
zsh completion: add _kernel-install
build-sys: link with librt if linking with libsd-daemon-int

In fba1ea0 'build: do not link everything with -lrt (and
therefore -pthread)' librt was removed from the list of
libraries. But libsd-daemon-internal also uses symbols from
librt and librt must thus be added everywhere where
libsd-daemon-interal is used, or otherwise linking might
fail:

/usr/bin/ld: ./.libs/libudev-core.a(sd-daemon.o): undefined reference to symbol 'mq_getattr@@GLIBC_2.3.4'
/usr/bin/ld: note: 'mq_getattr@@GLIBC_2.3.4' is defined in DSO /lib64/librt.so.1 so try adding it to the linker command line