[mux]: Fix mark_dhcp_packet

[theasianpianist]

Fixes #9374

Why I did it

Two issues prevent the mux systemd service from starting properly:

• The mux.service unit file contains two [Service] sections, making it invalid
• The mark_dhcp_packet.py file is not executable in the image

How I did it

• Consolidate the two [Service] sections by moving the ExecStartPre line for mark_dhcp_packet.py to the first section and removing the second.
• Make the mark_dhcp_packet.py file executable
• Also clean up mark_dhcp_packet.py
  • Remove unused imports
  • Fix spacing and line lengths to conform to PEP8

How to verify it

• Ensure that the mux service start properly:

admin@sonic:~$ systemctl status mux
● mux.service - MUX Cable Container
     Loaded: loaded (/lib/systemd/system/mux.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2021-11-24 20:16:51 UTC; 5min ago
    Process: 118541 ExecStartPre=/usr/local/bin/write_standby.py (code=exited, status=0/SUCCESS)
    Process: 120521 ExecStartPre=/usr/local/bin/mark_dhcp_packet.py (code=exited, status=0/SUCCESS)
    Process: 120649 ExecStartPre=/usr/bin/mux.sh start (code=exited, status=0/SUCCESS)
   Main PID: 120723 (mux.sh)
      Tasks: 2 (limit: 19193)
     Memory: 18.1M
     CGroup: /system.slice/mux.service
             ├─120723 /bin/bash /usr/bin/mux.sh wait
             └─120724 /usr/bin/python3 /usr/local/bin/container wait mux

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog

A picture of a cute animal (not mandatory but encouraged)

[lgtm-com]

This pull request fixes 3 alerts when merging 921091a into 80dfbc8 - view on LGTM.com

fixed alerts:

• 3 for Unused import