setjmp() gets the jmp_buf pointer from the wrong place (the place
where the return address is) in the shlib case, and uses it (only)
to fetch the current signal mask to address (return_address + 28).
This address is normally read-only (I hope), so the sigprocmask()
call has no effect except to return an error code.
\end{quote}
Submitted by: bde
bcf55ab2b8