C: fix cop_warnings overflow

provide 15 byte more space for cop_warnings.
Fixes asan ckwarn_common checks for #258, -O3 tests 156 174 175 184 192 207 232 243
(but breaks some other tests)
C warnsize-i258: fix how cop_warnings are stored

>5.10: 0|1|2 or as pascal string: STRLEN len + asciiz string of packed warnings bits.
2 new regressions: #184, #71
C warnsize-i258: fix STRLEN size packelement

we need a LONGSIZE pack template (STRLEN = ssize_t = long int),
but pack L is 4 on longsize=8 x86_64 unix systems, so we need UVSIZE J then.