rename libc to libjehanne; improve libposix

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

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

@@ -0,0 +1,24 @@
+/*
+ * 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>
+
+Rune*
+jehanne_runestrrchr(const Rune *s, Rune c)
+{
+	const Rune *r;
+
+	if(c == 0)
+		return jehanne_runestrchr(s, 0);
+	r = 0;
+	while(s = jehanne_runestrchr(s, c))
+		r = s++;
+	return (Rune *)r;
+}