core/unit: make JoinsNamespaceOf= implies the inverse dependency

Previously, even if a.service has JoinsNamespaceOf=b.service, the
inverse direction of reference was not introduced.
Hence, a.service is started earlier than b.service, the namespace will
not shared with b.service.
Also, even if a.service had the reference to b.service, b.service did not.
If b.service is freed earlier, then unit_clear_dependencies() does not clear
the reference from a to b, and will cause use-after-free on unit_free() for
a.service.

Let's make JoinsNamespaceOf=b.service in a.service implies the inverse
dependency, i.e. JoinsNamespaceOf=a.service for b.service. Then, we can safely
free b.service.

test/testsuite-23.units/testsuite-23-joins-namespace-of-5.service

@@ -3,4 +3,4 @@
 Type=oneshot
 MountAPIVFS=yes
 PrivateTmp=yes
-ExecStart=test ! -e /tmp/shared-private-file
+ExecStart=test -e /tmp/shared-private-file

test/testsuite-23.units/testsuite-23-joins-namespace-of-8.service

@@ -4,6 +4,6 @@ Type=notify
 NotifyAccess=all
 MountAPIVFS=yes
 PrivateTmp=yes
-ExecStartPre=test ! -e /tmp/shared-private-file-x
+ExecStartPre=test -e /tmp/shared-private-file-x
 ExecStartPre=test ! -e /tmp/hoge
 ExecStart=/bin/bash -c 'touch /tmp/shared-private-file-y && systemd-notify --ready && sleep infinity'

test/testsuite-23.units/testsuite-23-joins-namespace-of-9.service

@@ -7,5 +7,5 @@ Type=oneshot
 MountAPIVFS=yes
 PrivateTmp=yes
 ExecStart=test ! -e /tmp/shared-private-file-x
-ExecStart=test -e /tmp/shared-private-file-y
+ExecStart=test ! -e /tmp/shared-private-file-y
 ExecStart=test ! -e /tmp/hoge