dhclient-script: T1987: Multiple fixes in dhclient-script

This changeset contains multiple changes in structure, logic, and bugfixes for dhclient-script. It should provide better compatibility with new Debian versions and flexibility in controlling and changing VyOS-related functions.
1. Structure change:
   * All VyOS-related functionality was moved from dhclient-script itself to separated hook files.
   * Old vyatta-dhclient-hook was moved from vyatta-cfg to vyos-1x.
   * This change allows discard dhclient-script replacing and use the original one from Debian without any changes. So, we do not need to track all changes in upstream so carefully.
   * To provide compatibility between original dhclient-script and VyOS, two internal commands/functions are repaced in hooks: ip and make_resolv_conf. So, in all places where used ${ip} or make_resolv_conf, actually using VyOS-tuned functions instead original.
   * `ip` function is a wrapper, which automatically chooses what to use: transparently pass a command to /usr/sbin/ip, change a route in kernel table or FRRouting config via vtysh.
   * `make_resolv_conf` function main logic was copied from current VyOS implementation and use vyos-hostsd-client for making changes
2. Added:
   * Logging. Now is possible to log all changes, what is doing by dhclient-script. Logs can be saved to the journal and displayed in stderr (for debugging purposes). By default, logging to the journal is enabled (at least for some time) to provide a way to collect enough information in case if some bug in this new implementation will be found. This can be changed in the 01-vyos-logging file.
3. Fixed/Changed:
   * If DHCP lease was expired, released or dhclient was stopped, dhclient-script will try to delete default route from this lease.
   * Instead of blindly killing all dhclients in case if FRRouting daemon is not running, now used more intelligent logic:
     * dhclients are stopping natively (with all triggers processing), instead of killing;
     * dhclient-script will not kill parent dhclient process. This allows to fix the problem when systemd inform about failing to rise up interfaces at early boot stages (used in Cloud-init images);
     * dhclient-script will not touch dhclients, which are not related to the current interface or IP protocol version.
   * For getting FRRouting daemon status used native way via watchfrr.sh, instead of the previous trick with vtysh accessibility.
   * before adding a new route to FRRouting configuration, this route will be deleted from the kernel (if it is presented there). This allows to properly replace routes, added at early boot stages, when FRR not available.
   * Routes in FRRouting are adding with "tag 210". This allows protecting static routes, added via CLI, from deletion when old routes are deleting by DHCP.
   * DNS servers will be reconfigured only when $new_domain_name_servers are not the same as $old_domain_name_servers. Previously, this was done during each RENEW procedure.
   * Replacing MTU for preconfigured one was changed to Python (via vyos.config). The previous version with vyatta-interfaces.pl was obsoleted and seems to be broken.

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