bsd_to_linux_signal() mapped every FreeBSD signal above 31 except the
SIGPWR stand-in as a real-time signal: SIGTHR became -1, 34..64
aliased Linux 1..31, and 99..126 exceeded LINUX_SIGRTMAX. Signal 0
indexed the table at -1. bsd_to_linux_sigset() shifted by these, so
linprocfs showed Linux signal 63 caught by every native threaded
process, and PR_GET_PDEATHSIG with no signal set read before the
table. Found with KUBSAN.
Return 0 for these, as the table already does for SIGEMT and SIGINFO.
A child killed by one would then read as exit status 0 in its wait
status and SIGCHLD siginfo, so report SIGKILL there.
siginfo_to_lsiginfo() also left si_status unconverted for CLD_DUMPED
and CLD_TRAPPED, so a core dump on SIGBUS reported Linux SIGUSR1.