When NM is restarted and a new object-manager is created, ensure that
signal handlers are disconnected from the old one.

Fixes the following:
  assertion failed: (object_manager == priv->object_manager)

 #0  __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
 #1  __GI_abort () at abort.c:90
 #2  g_assertion_message (domain=domain@entry=0x7fcac0b845ff "libnm", file=file@entry=0x7fcac0b84c95 "libnm/nm-client.c", line=line@entry=2506, func=func@entry=0x7fcac0b863a0 <__func__.34881> "name_owner_changed", message=message@entry=0x7fcac20b05f0 "assertion failed: (object_manager == priv->object_manager)") at gtestutils.c:2429
 #3  g_assertion_message_expr (domain=domain@entry=0x7fcac0b845ff "libnm", file=file@entry=0x7fcac0b84c95 "libnm/nm-client.c", line=line@entry=2506, func=func@entry=0x7fcac0b863a0 <__func__.34881> "name_owner_changed", expr=expr@entry=0x7fcac0b856a0 "object_manager == priv->object_manager") at gtestutils.c:24...