systemd: directly notify systemd instead of using sd_notify

The sd_notify method is used to tell systemd when libvirtd
has finished starting up. All it does is send a datagram
containing the string parameter to systemd on a UNIX socket
named in the NOTIFY_SOCKET environment variable. Rather than
pulling in the systemd libraries for this, just code the
notification directly in libvirt as this is a stable ABI
from systemd's POV which explicitly allows independant
implementations:

See "Reimplementable Independently" column in the
"$NOTIFY_SOCKET Daemon Notifications" row:

https://www.freedesktop.org/wiki/Software/systemd/InterfacePortabilityAndStabilityChart/

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1314881

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

libvirt.spec.in

@@ -79,7 +79,6 @@
 %define with_firewalld     0%{!?_without_firewalld:0}
 %define with_libssh2       0%{!?_without_libssh2:0}
 %define with_wireshark     0%{!?_without_wireshark:0}
-%define with_systemd_daemon 0%{!?_without_systemd_daemon:0}
 %define with_pm_utils      1
 
 # Finally set the OS / architecture specific special cases
@@ -133,7 +132,6 @@
 # Fedora has systemd, libvirt still used sysvinit there.
 %if 0%{?fedora} || 0%{?rhel} >= 7
     %define with_systemd 1
-    %define with_systemd_daemon 1
     %define with_pm_utils 0
 %endif
 
@@ -268,9 +266,6 @@ BuildRequires: python
 %if %{with_systemd}
 BuildRequires: systemd-units
 %endif
-%if %{with_systemd_daemon}
-BuildRequires: systemd-devel
-%endif
 %if %{with_xen} || %{with_libxl}
 BuildRequires: xen-devel
 %endif
@@ -1061,12 +1056,6 @@ rm -rf .git
     %define arg_wireshark --without-wireshark-dissector
 %endif
 
-%if %{with_systemd_daemon}
-    %define arg_systemd_daemon --with-systemd-daemon
-%else
-    %define arg_systemd_daemon --without-systemd-daemon
-%endif
-
 %if %{with_pm_utils}
     %define arg_pm_utils --with-pm-utils
 %else
@@ -1157,7 +1146,6 @@ rm -f po/stamp-po
            --with-driver-modules \
            %{?arg_firewalld} \
            %{?arg_wireshark} \
-           %{?arg_systemd_daemon} \
            %{?arg_pm_utils} \
            --with-nss-plugin \
            %{arg_packager} \