ShellPkg-UefiShellLib: Add a function to fully-qualify paths

ShellPkg-UefiShellLib: Add a function to fully-qualify paths

Add a function to return a clean, fully-qualified version of some path.

This function handles a (possibly "dirty") input path that may or may
not include a file system reference.

If it does not include a file system reference, then if the input path
does not begin with a forward or backward slash, then the input path is
relative to the current working directory of the current file system.
Otherwise, it is an absolute path within the current file system.

If it does include a file system reference, it may be a reference to the
current or some other file system.  If the file system reference is not
immediately followed by a forward or backward slash, then the input path
is relative to the current working directory of the given file system.
Otherwise, it is an absolute path within the given file system.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jim Dailey <jim_dailey@dell.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>