[FREELDR] Other enhancements.

[FREELDR] Other enhancements.

- Add optional arguments BootDrive and BootPartition to
  ChainLoadBiosBootSectorCode() so as not to modify explicitly the
  FrldrBootDrive and FrldrBootPartition variables, that should remain
  purely internal.

- Implement ChainLoadBiosBootSectorCode() for x64.

- Get rid of the machine-specific DiskGetBootPath(), and instead do its
  job only once in the machine-specific InitializeBootDevices() (or in
  MachInit() for PPC).
  Cache the result of this operation into the globally-accessible
  FrldrBootPath buffer. This avoids the unneeded calls to (Mach)DiskGetBootPath()
  we used to do before.
  Also remove the separate distinction between the PC and XBOX versions
  of this functionality.

- Move the PC-specific DiskIsDriveRemovable() and DiskGetBootPath()
  as well as the disk-IO-error functionality, back into the
  corresponding PC-arch files.

- Simplify IniFileInitialize(), getting rid of IniOpenIniFile().