Wait for thread loop to return when exiting

Exit with an error if config command fails because 'PyYAML' is missing.

Run pim-dm in foreground

exit class can be used to handle signals and do proper cleaning before killing
a process

As running stop and then start is the default behavior for daemon managing tools
restart command, the current restart command looks pointless.

On stop command call Main.stop() runs
`remove_interface("*", pim=True, membership=True, ipv4=True, ipv6=True)`.

Therefore if a PIM interface was added for network interface eth0, the call
`remove_interface('eth0', True, True)` will evaluate the condition
`membership and not membership_interface` as True and the PIM interface will
never be removed.

Furthermore
`remove_interface("*", pim=True, membership=True, ipv4=True, ipv6=True)` will
do calls like `remove_interface('lo', True, True)` then calling
`remove_virtual_interface('lo')` and the line
`index = self.vif_name_to_index_dic.pop(interface_name, None)` will cause a
KeyError when running
`del self.vif_name_to_index_dic[self.vif_index_to_name_dic[index]]` or
`mif_index = self.vif_name_to_index_dic.pop(interface_name)`.

Return a tuple for vrfs instead of a list

update github workflow

Allow to run PIM, IGMP or MLD on working interfaces even if it failed for other
interfaces listed in the configuration file.

Reading install_requires from requirements.txt makes it impossible to install
from pypi archive by simply running `python setup.py install` as
requirements.txt is not part of the archive.

Fix the following error:

```
Exception in thread Thread-4:
Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 1166, in run
    self.function(*self.args, **self.kwargs)
  File "/usr/local/lib/python3.7/dist-packages/pimdm/InterfacePIM.py", line 152, in send_hello
    self.send(packet.bytes())
  File "/usr/local/lib/python3.7/dist-packages/pimdm/InterfacePIM6.py", line 86, in send
    super().send(data=data, group_ip=group_ip)
  File "/usr/local/lib/python3.7/dist-packages/pimdm/InterfacePIM.py", line 113, in send
    if self.drop_packet_type is not None:
AttributeError: 'InterfacePim6' object has no attribute 'drop_packet_type'
```

Add command to list instances && fix issue of -mvr and -uvrf being mutually exclusive && fix state refresh deadlock && include originator state on -ls

* First try IPv6 support... based on HPIM-DM IPv6 commit

* fix membership removal of interface (IGMP/MLD)

* Add support for unicast and multicast VRFs && remove some commented code && IGMP/MLD snake case global variables

Bump setup-python from v1 to v2

Allow enabling/disabling StateRefresh on previously disabled/enabled PIM-DM interfaces without having to remove them