core, shared: in deserializing, match same files reached via different paths

core, shared: in deserializing, match same files reached via different paths

When dbus.socket is updated like this:
-ListenStream=/var/run/dbus/system_bus_socket
+ListenStream=/run/dbus/system_bus_socket
... and daemon-reload is performed, bad things happen.
During deserialization systemd does not recognize that the two paths
refer to the same named socket and replaces the socket file with a new
one. As a result, applications hang when they try talking to dbus.

Fix this by finding a match not only when the path names are equal, but
also when they point to the same inode.
In socket_address_equal() it is necessary to move the address size
comparison into the abstract sockets branch. For path name sockets the
comparison must not be done and for other families it is redundant
(their sizes are constant and checked by socket_address_verify()).

FIFOs and special files can also have multiple pathnames, so compare the
inodes for them as well. Note that previously the pathname checks used
streq_ptr(), but the paths cannot be NULL.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1186018