signals: a blocked signal stays pending instead of killing

sigsend() ended a process for a default-fatal signal (SIGUSR1, SIGTERM,
SIGINT...) on arrival, whatever its signal mask said. Blocking is
exactly the promise that it will not: the signal waits until it is
unblocked, or until the program takes it some other way -- which is
what signalfd is, and how this was found.

A blocked signal is now only made pending. If it is later unblocked
while still pending, deliver_signals() applies the default action
then. SIGKILL cannot be blocked, as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/core/defs.h

@@ -530,6 +530,7 @@ int             proc_pgid(int pid);
 int             proc_setpgid(int pid, int pgid);
 int             proc_setsid(void);
 int             sigsend(int pid, int sig);
+int             sig_default_kills(int sig);
 int             proc_exists(int);
 int             proc_num(void);
 struct procinfo;