pack(): avoid << of negative values

pack(): avoid << of negative values

Treat the string as U8* rather than char* when doing all the
bit shifts for uuencode. That stops these warnings under ASan:

    pp_pack.c:1890:34: runtime error: left shift of negative value -127
    pp_pack.c:1891:34: runtime error: left shift of negative value -126
    pp_pack.c:1899:34: runtime error: left shift of negative value -1
    pp_pack.c:1900:30: runtime error: left shift of negative value -31