Use gnulib's c-ctype.h, not <ctype.h>.

Use gnulib's c-ctype.h, not <ctype.h>.

# Convert uses of isspace to c_isspace, isdigit to c_isdigit, etc.
re=$(man isspace|grep is.....,.is|sed 's/ -.*//' \
  |tr -s ', \n' \||sed 's/^|//;s/|$//')
git grep -l -E "$re"|grep -Ev 'Chan|gnulib' \
  |xargs perl -pi -e 's/\b('"$re"')\b/c_$1/g'
# Remove all uses of to_uchar
git grep -l to_uchar|xargs perl -pi -e 's/to_uchar\((.*?)\)/$1/g'
* src/util.h (to_uchar): Remove definition.
(TOLOWER): Remove definition.
(__virMacAddrCompare): Use c_tolower, not TOLOWER.
Globally:
  Where needed, change <ctype.h> to <c-ctype.h>.
  Remove unnecessary inclusion of <ctype.h>.
Ensure the global changes are never needed again:
* Makefile.maint (sc_avoid_ctype_macros): Prohibit use of ctype
macros.   Recommend c-ctype.h instead.
(sc_prohibit_c_ctype_without_use): New rule.
(sc_prohibit_ctype_h): New rule.  Disallow use of <ctype.h>.