libiberty: {count,dup,write}argv: constify argv input slightly

libiberty: {count,dup,write}argv: constify argv input slightly

Would be more useful if we could use "const char * const *", but there's
a long standing bug where gcc warns about incompatible pointers when you
try to pass in "char **".  We can at least constify the array itself as
gcc will not warn in that case.