Cygwin: is_unicode_equiv: implement Unicode equivalence class check

Cygwin: is_unicode_equiv: implement Unicode equivalence class check

is_unicode_equiv compares two UTF-32 values and returns 1 if
both are member of the same Unicode equivalence class, 0 otherwise.

Note that this function only works with precomposed characters
per Unicode normalization form C.  It doesn't handle decomposed
characters, just like its counterpart in glibc.  I.e., equivalence
class comparison using decomposed chars won't work.  Example:

  fnmatch("[=n=]", "ñ") == 0
  fnmatch("[=ñ=]", "n") == 0

but

  fnmatch("[=n=]", "n\x0303") == 1
  fnmatch("[=n\x0303=]", "n") == 1
  fnmatch("[=n\x0303=]", "n\x0303") == 1

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>