user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,37 @@
+#include <assert.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/resource.h>
+#include <unistd.h>
+#include <limits.h>
+
+int main() {
+	errno = 0;
+	int original_priority = nice(0);
+
+	assert(original_priority != -1 || !errno);
+
+	errno = 0;
+	int ret = nice(1);
+
+	if(ret == -1 && errno) {
+		fprintf(stderr, "%s", strerror(errno));
+		exit(1);
+	}
+
+	errno = 0;
+	int new_priority = nice(0);
+
+	assert(new_priority != -1 || !errno);
+	assert(new_priority == original_priority + 1);
+
+	errno = 0;
+	new_priority = nice(INT_MAX - new_priority);
+
+	assert(new_priority != -1 || !errno);
+	assert(new_priority != INT_MAX);
+
+	return 0;
+}