[SysVars] Add conversions to get remote-unit info

[tonhuisman]

From a forum request

Features:

• Add conversions to retrieve information about a remote unit, available/visible via ESPEasy P2P:
  • %c_uname%(<unit>) Get the name of the remote unit, empty if not available.
  • %c_uage%(<unit>) Get the age of the remote unit, -1 if not available, 0 or positive value is 'last-seen' in seconds.
  • %c_ubuild%(<unit>) Get the buildnumber of the remote unit. 0 if not available.
  • %c_ubuildstr%(<unit>) Get the build of the remote unit.
    When newer than build 20200, it is formatted like a date: 20240421, as displayed on the Node List of the ESPEasy main page, older builds are in the range 20000..20199 (theoretically).
  • %c_uload%(<unit>) Get the CPU load of the remote unit.
  • %c_utype%(<unit>) Get the numeric ESP/etc type. (see docs for list)
  • %c_utypestr%(<unit>) Get the ESP/etc Type name as shown on the Main tab.
• Updated documentation

TODO:

• Confirmation by requester (confirmed)
• Update EasyColorCode with new variables (and some corrections)