user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/tests/posix/pause.c

@@ -0,0 +1,49 @@
+#include <assert.h>
+#include <signal.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <errno.h>
+#include <sys/wait.h>
+
+static void noop(int x) {
+	(void)x;
+}
+
+int main() {
+	signal(SIGUSR1, &noop);
+
+	int pid;
+	switch(pid = fork()) {
+	case -1:
+		perror("fork");
+		abort();
+	case 0:
+		pause();
+		assert(errno == EINTR);
+		return 0;
+	default:
+		while (1) {
+			usleep(100);
+			kill(pid, SIGUSR1);
+			usleep(100);
+			int status;
+
+			errno = 0;
+			if (waitpid(-1, &status, WNOHANG) <= 0) {
+				if (errno && errno != EAGAIN) {
+					perror("wait");
+					kill(pid, SIGKILL);
+				}
+				continue;
+			}
+
+			if (!WIFEXITED(status)) {
+				printf("wait returned %x\n", status);
+				abort();
+			}
+
+			return WEXITSTATUS(status);
+		}
+	}
+}