linux: fix FreeBSD to Linux signal conversion

[nprice]

Summary

bsd_to_linux_signal() looks up signals 1..31 in a table and treats
everything else but the SIGPWR stand-in as a real-time signal,
sig - SIGRTMIN + LINUX_SIGRTMIN, or sig - 33.  That is right only for
65..97, so the signals with no Linux counterpart came out as:

  32 (SIGTHR)    -> -1
  33 (SIGLIBRT)  -> 0
  34..64         -> 1..31, unrelated Linux signals
  99..126        -> 66..93, past LINUX_SIGRTMAX

Signal 0 indexed the table at -1.

- bsd_to_linux_sigset() turned SIGTHR's -1 into a negative shift,
  which on amd64 sets Linux signal 63, so linprocfs showed it caught
  for every native threaded process.  KUBSAN reported the shift.
- prctl(PR_GET_PDEATHSIG) with no signal set returned whatever int
  preceded the table.
- A Linux parent of a native child killed by SIGEMT or SIGLIBRT saw
  a normal exit, so bash reported $? = 0, success.  SIGTHR gave wait
  status 0xffffffff ($? = 255), and 34..64 unrelated signals.

Return 0 for these, as the table already does for SIGEMT and SIGINFO,
and report a child killed by a signal that maps to 0 as killed by
SIGKILL rather than as a normal exit.

siginfo_to_lsiginfo() also left si_status unconverted for CLD_DUMPED
and CLD_TRAPPED, so a core dump on SIGBUS reported 10, Linux SIGUSR1.