system: op-mode: T3334: allow delayed getty restart when configuring serial ports

• Created op-mode command "restart serial console"
• Relocated service control to vyos.utils.serial helpers, used by conf- and op-mode serial console handling
  • Checking for logged-in serial sessions that may be affected by getty reconfig
  • Warning the user when changes are committed and serial sessions are active, otherwise restart services as normal. No prompts issued during commit, all config gen/commit steps still occur except for the service restarts (everything remains consistent)
  • To apply committed changes, user will need to run "restart serial console" to complete the process or reboot the whole router
  • Added additional flags and target filtering for generic use of helpers.