Fix portability problem in techo()

Fix portability problem in techo()

ISCONTROL chechks if a value is between 0 and 0x1f or
between 0x80 and 0x9f. Char signess depends of architecture
and compiler, so in some environment the second case is
always false (and wrong), Techo() calls ISCONTROL with a
char variable, whose type cannot be changed because tpuc()
expects a pointer to char, so the solution is to insert a
cast in the call to ISCONTROL.