man: Fix XDG_RUNTIME_DIR variable reference.
build-sys: Clarify some systemd sub-component variable/define names.

We currently use the term SYSTEMD when referring to libsystemd-login
and JOURNAL when referring to libsystemd-journal.

I will be shortly adding support for libsystemd-daemon and in
preparation I figured it would be a good idea to clarify the names
used currently before adding another!
build-sys: Add support for newer systemd without compatibility libs

In newer versions of systemd some libraries were combined for the sake of
general simplicity.

This change checks against the newer name first and avoids separate pkgconfig
checks if it's found. We probably want to keep support for the older library
names for some time. systemd does allow for the shipping of compatibility
pkgconfig files to not break downstream code like ourselves which is why this
likely hasn't been "fixed" until now.

With this change we no longer rely on systemd having been built with those
compatibility pkgconfig files.
socket-server: Add support for systemd socket activation.

This adds support to module-native-protocol-unix to take over already
listening sockets passed in via socket activation (e.g. from systemd)

Most of the code is isolated to socket-server but some cleanup code also
had to be tweaked to ensure we do not overzealously close open fds.
launch: Avoid specifically starting PA and rely on autospawn/socket activation

This --start is patched out in several downstreams to allow users to easily
disable PA by simply disabling autospawn.

If autospawn is enabled, then the first pactl command will start it and if not
it will fail and the script will exit.

When switching to systemd socket activation, we very much do not want to
start PA manually here. We could replace it with a
  systemctl --user start pulseaudio
but really it just makes sense to rely on the socket activation as this
should apply equally to non-systemd setups which use PA's own autospawn.
launch: Add systemd units for launching pulseaudio user instances
launch: Disable autospawn by default when systemd daemon support is enabled.

When enabled, this method is prefered over pulseaudio's built in
systems so we should try our best to ensure that it cannot be spawned
outside of the mechanisms desired.

Packagers should call 'systemctl --global enable pulseaudio.socket' to
enable the socket for all users, or alternatively ship an enabling
symlink in /usr/lib/systemd/user/sockets.target.wants/ folder. It may
also make sense for distributions to add in a ConditionNNN= line to the
socket unit if they have a downstream mechanism for enabling or
disabling pulseaudio.

If individual users wish to opt out of this vendor (or administrator)
decision, they can call 'systemctl --user mask pulseaudio.socket'
build-sys: Minor cosmetic tidyup of configure summary