StdLib/Include: Minor changes in preparation for Interactive I/O (TTY) functionality.

StdLib/Include: Minor changes in preparation for Interactive I/O (TTY) functionality.

StdLib/Include/
  stdlib.h
    Update MB_CUR_MAX to 3, the max. size of a MBCS character.
    Improve comments for wcstombs()
    Declare implementation-specific MBCS utility functions:
      OneWcToMcLen()    Determine the number of bytes needed to represent
                        a Wide character as a MBCS character.
      EstimateWtoM()    Determine the number of bytes needed to represent
                        a Wide character string as a MBCS string.
      CountMbcsChars()  Determine the number of characters in a MBCS string.

  wchar.h
    Improve comments.

StdLib/Include/sys/
  fcntl.h
    Define new Open flags.
    Remove obsolete and commented-out lines.

  termios.h
    Add Intel Copyright notice and Open-Source License.
    Change c_cc index macros into enumerated values.
    Clean up flag definitions for visual alignment as well as UEFI relevance.
    Move c_ispeed and c_ospeed termios members to end for better alignment.
    Comment out declarations for functions not yet implemented.
    Add an enum defining values for the UEFI extended function keys.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: erik.c.bjorge@intel.com
Reviewed-by: jaben.carsey@intel.com
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13741 6f19259b-4bc3-4df7-8a09-765794883524