Support multi-byte --transform='...\L...' etc

Support multi-byte --transform='...\L...' etc

Support upcasing and downcasing in multi-byte locales.
* gnulib.modules: Add c32rtomb, c32tolower, c32toupper,
mbrtoc32-regular.
* src/transform.c: Do not include ctype.h.  Include mcel.h.
(stk, stk_init): Move up.
(run_case_conv): Return void, not char *.  Append result to
stk directly; this avoids the need for a separate allocation.
All callers changed.  Do not assume a single-byte locale.
* tests/xform04.at: New test.
* tests/Makefile.am (TESTSUITE_AT):
* tests/testsuite.at: Add it.