systemd-activate: pass environment variables through

I need this to test half-installed socket-activated python
script, which requires PYTHONPATH and LD_LIBRARY_PATH set.
I assume that other people might find it useful to.

-E VAR passes through VAR from the environment, while
-E VAR=value sets VAR=value.

systemd-activate -E PYTHONPATH=/var/tmp/inst1/usr/lib64/python3.3/site-packages -E LD_LIBRARY_PATH=/var/tmp/inst1/usr/lib -l 2000 python3 -c 'from systemd.daemon import listen_fds; print(listen_fds())'
systemd-python: add systemd.daemon wrapping sd-daemon

Please see the documentation (e.g. pydoc3 systemd.daemon) for full
description. As usual, systemd._daemon wraps the raw interface, while
systemd.daemon provides the more pythonic API. sd_listen_fds,
sd_booted, sd_is_fifo, sd_is_socket, sd_is_socket_unix,
sd_is_socket_inet, sd_is_mq, and SD_LISTEN_FDS_START are currently
wrapped.
man: fix grammar
core: keep mountinfo .mounts until late shutdown

.mount units coming from /proc/self/mountinfo file are
unmounted after local-fs.target is reached during shutdown.

Problem: .mount units popping up in mountinfo file are
added to systemd without any dependency. For that reason,
they are the first one to be unmounted during shutdown.
Whichever program mounted the file system deserves a
chance to also unmount it. This patch ensures that
/proc/self/mountinfo units will be unmounted after
local-fs.target during shutdown (if they haven't been
unmounted already)
units: ignore systemd-sysctl on shutdown

No need to try to stop systemd-sysctl on shutdown as
this service doesn't have ExecStop= anyways.
smack-setup: extract rule writing into a separate function

Check all errors.
smack-setup: enable Smack/CIPSO mapping

CIPSO is the Common IP Security Option, an IETF standard for setting
security levels for a process sending packets. In Smack kernels,
CIPSO headers are mapped to Smack labels automatically, but can be changed.

This patch writes label/category mappings from /etc/smack/cipso/ to
/sys/fs/smackfs/cipso2. The mapping format is "%s%4d%4d"["%4d"]...

For more information about Smack and CIPSO, see:
  https://kernel.org/doc/Documentation/security/Smack.txt
TODO: uses for SO_REUSEPORT

The new socket option SO_REUSEPORT would enable some new functionality;
add it to TODO.
core: reuse the same /tmp, /var/tmp and inaccessible dir

All Execs within the service, will get mounted the same
/tmp and /var/tmp directories, if service is configured with
PrivateTmp=yes. Temporary directories are cleaned up by service
itself in addition to systemd-tmpfiles. Directory which is mounted
as inaccessible is created at runtime in /run/systemd.
timer: fix grammar in message