FatPkg EnhancedFatDxe: Use safe string functions

FatPkg EnhancedFatDxe: Use safe string functions

Unsafe string functions are replaced with safe ones.

Safe string functions will assert if DestMax is not greater than
StrnLenS(Source, DestMax). Therefore, additional assert for checking the
size of source and destination buffers can be removed.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
(based on FatPkg commit 2cb92b4f19b096daf133d6501afa13e5a85062c5)

[jordan.l.justen@intel.com: Use script to relicense to 2-clause BSD]
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Mark Doran <mark.doran@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>