[FREELDR] Minor enhancements for disk/partition boot and UI.

[FREELDR] Minor enhancements for disk/partition boot and UI.

- Factor out disk & partition opening in LoadAndBootPartition() and
  LoadAndBootDrive() into a LoadAndBootPartitionOrDrive() helper.

- Use ARC paths and ArcOpen()/ArcRead() instead of calling the
  machine-specific BIOS-oriented DiskGetPartitionEntry() and
  MachDiskReadLogicalSectors() functions to open the disk/partition
  and read their boot sectors.

- Don't forget to close the opened boot sector file in LoadAndBootBootSector().

- Add assertions for DiskReadBufferSize in PcMemFinalizeMemoryMap()
  and EnumerateHarddisks().

- x86/amd64 only:
  * Add a DisableA20 helper for disabling the A20 line, before rebooting
    back, or chain-load a boot sector, into 16-bit world.
    Also pulse the output port of the keyboard controller to clear out
    its state after having set the state of the A20 line.

  * In addition to disabling the A20 line when rebooting or chain-loading
    a boot sector, reset the video back to 80x25 text mode.

- Reset the cursor position back to the origin when initializing or
  terminating the TUI.