Implement signal injection

Implement signal injection

This extends the fault injection capability with :signal=SIG option
which injects a signal on entering each syscall from the specified set.

:signal and :error options are complementary, if they are both specified
the syscall will be fault injected as usual and the specified signal
will be delivered to the tracee.

* defs.h (struct fault_opts): Change the type of err field to int16_t,
add signo field.
(trace_syscall): Add a pointer argument.
* qualify.c: Include "nsig.h".
(parse_fault_token): Handle signal= option.
(qualify_fault): Update default fault_opts.
* strace.c (trace): Forward signal number from trace_syscall
to ptrace_restart(PTRACE_SYSCALL).
* syscall.c (inject_syscall_fault_entering): Add pointer argument,
save there the signal number specified by fault options.  Do not inject
a syscall fault unless instructed by fault options.
(update_syscall_fault_exiting): Update the error code injection check.
(trace_syscall_entering): Add pointer argument, forward it to
inject_syscall_fault_entering.
(trace_syscall): Add pointer argument, forward it to
trace_syscall_entering.

Signed-off-by: Seraphime Kirkovski <kirkseraph@gmail.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>