ShellPkg: UpdateStdInStdOutStdErr(): append BOM to new unicode file

ShellPkg: UpdateStdInStdOutStdErr(): append BOM to new unicode file

The >> operator redirects stdout to a file, using append mode and unicode
encoding. Write the BOM when redirection happens to a new file (which
starts out empty).

This makes the >> operator behave similarly to the > operator, when the
redirection target doesn't exist originally:

  OutUnicode && OutAppend && FileSize == 0 // >> to new unicode file
vs.
  OutUnicode && !OutAppend                 // >  to any unicode file

(Note that (FileSize == 0) is equivalent to "new file" in this context,
due to the earlier "Check that filetypes (Unicode/Ascii) do not change
during an append".)

Reported-by: Lowell Dennis <Lowell_Dennis@Dell.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15725 6f19259b-4bc3-4df7-8a09-765794883524