use 0 instead of NULL for null pointer constants

and thereby remove otherwise-unnecessary inclusion of stddef.h

src/signal/sighold.c

 #include <signal.h>
-#include <stddef.h>
 
 int sighold(int sig)
 {
@@ -7,5 +6,5 @@ int sighold(int sig)
 
 	sigemptyset(&mask);
 	if (sigaddset(&mask, sig) < 0) return -1;
-	return sigprocmask(SIG_BLOCK, &mask, NULL);
+	return sigprocmask(SIG_BLOCK, &mask, 0);
 }

src/signal/sigignore.c

 #include <signal.h>
-#include <stddef.h>
 
 int sigignore(int sig)
 {
@@ -8,5 +7,5 @@ int sigignore(int sig)
 	sigemptyset(&sa.sa_mask);
 	sa.sa_handler = SIG_IGN;
 	sa.sa_flags = 0;
-	return sigaction(sig, &sa, NULL);
+	return sigaction(sig, &sa, 0);
 }

src/signal/siginterrupt.c

-#include <stddef.h>
 #include <signal.h>
 
 int siginterrupt(int sig, int flag)
 {
 	struct sigaction sa;
 
-	sigaction(sig, NULL, &sa);
+	sigaction(sig, 0, &sa);
 	if (flag) sa.sa_flags &= ~SA_RESTART;
 	else sa.sa_flags |= SA_RESTART;
 
-	return sigaction(sig, &sa, NULL);
+	return sigaction(sig, &sa, 0);
 }

src/signal/sigrelse.c

 #include <signal.h>
-#include <stddef.h>
 
 int sigrelse(int sig)
 {
@@ -7,5 +6,5 @@ int sigrelse(int sig)
 
 	sigemptyset(&mask);
 	if (sigaddset(&mask, sig) < 0) return -1;
-	return sigprocmask(SIG_UNBLOCK, &mask, NULL);
+	return sigprocmask(SIG_UNBLOCK, &mask, 0);
 }

src/signal/sigset.c

 #include <signal.h>
-#include <stddef.h>
 
 void (*sigset(int sig, void (*handler)(int)))(int)
 {
@@ -11,7 +10,7 @@ void (*sigset(int sig, void (*handler)(int)))(int)
 		return SIG_ERR;
 	
 	if (handler == SIG_HOLD) {
-		if (sigaction(sig, NULL, &sa_old) < 0)
+		if (sigaction(sig, 0, &sa_old) < 0)
 			return SIG_ERR;
 		if (sigprocmask(SIG_BLOCK, &mask, &mask) < 0)
 			return SIG_ERR;

src/signal/sigwait.c

 #include <signal.h>
-#include <stddef.h>
 
 int sigwait(const sigset_t *restrict mask, int *restrict sig)
 {
 	siginfo_t si;
-	if (sigtimedwait(mask, &si, NULL) < 0)
+	if (sigtimedwait(mask, &si, 0) < 0)
 		return -1;
 	*sig = si.si_signo;
 	return 0;

src/signal/sigwaitinfo.c

 #include <signal.h>
-#include <stddef.h>
 
 int sigwaitinfo(const sigset_t *restrict mask, siginfo_t *restrict si)
 {
-	return sigtimedwait(mask, si, NULL);
+	return sigtimedwait(mask, si, 0);
 }