rename libc to libjehanne; improve libposix

This is to avoid conflicts between standards' assumptions and Jehanne's choices

sys/src/lib/jehanne/port/strncat.c

@@ -0,0 +1,28 @@
+/*
+ * This file is part of the UCB release of Plan 9. It is subject to the license
+ * terms in the LICENSE file found in the top-level directory of this
+ * distribution and at http://akaros.cs.berkeley.edu/files/Plan9License. No
+ * part of the UCB release of Plan 9, including this file, may be copied,
+ * modified, propagated, or distributed except according to the terms contained
+ * in the LICENSE file.
+ */
+
+#include <u.h>
+#include <libc.h>
+
+char*
+jehanne_strncat(char *s1, const char *s2, int32_t n)
+{
+	char *os1;
+
+	os1 = s1;
+	while(*s1++)
+		;
+	s1--;
+	while(*s1++ = *s2++)
+		if(--n < 0) {
+			s1[-1] = 0;
+			break;
+		}
+	return os1;
+}