add setdomainname syscall, fix getdomainname (previously a stub)

include/unistd.h

@@ -165,6 +165,7 @@ int getpagesize(void);
 int getdtablesize(void);
 int sethostname(const char *, size_t);
 int getdomainname(char *, size_t);
+int setdomainname(const char *, size_t);
 int setgroups(size_t, const gid_t []);
 char *getpass(const char *);
 int daemon(int, int);

src/misc/getdomainname.c

+#define _GNU_SOURCE
 #include <unistd.h>
 #include <sys/utsname.h>
 #include <string.h>
+#include <errno.h>
 
 int getdomainname(char *name, size_t len)
 {
-	*name = 0;
+	struct utsname temp;
+	uname(&temp);
+	if (!len || strlen(temp.domainname) >= len) {
+		errno = EINVAL;
+		return -1;
+	}
+	strcpy(name, temp.domainname);
 	return 0;
 }

src/misc/setdomainname.c

+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+int setdomainname(const char *name, size_t len)
+{
+	return syscall(SYS_setdomainname, name, len);
+}