SIGTRAP is useful for debugging but it has particular interactions with
the debugger that make it difficult for a process to use the existing
mechanism to provide a fall-back codepath for Capsicum failures.
On Linux, seccomp-bpf can raise SIGSYS to allow this on blocked calls.
After discussion it was decided that we should add a new signal, rather
than overload SIGSYS, for this. This makes it easy to not receive a
signal for system calls that don't exist at all, only ones that exist
but are not permitted in the current context.