Problems reported by Daniel Richard G. in:
http://lists.gnu.org/archive/html/bug-gnulib/2015-09/msg00050.html
* lib/c-ctype.h (_C_CTYPE_CNTRL): Rewrite in terms of
the C standard escapes and _C_CTYPE_OTHER_CNTRL.
(_C_CTYPE_OTHER_CNTRL): New macro.
* tests/test-c-ctype.c (test_all): Test from CHAR_MIN, not
from SCHAR_MIN, as the functions are defined only from values
promoted from char or from unsigned char, not necessarily from
signed char.